First-off, congratulation! (on starting your poverty training early!).

These rigs aren't cheap, but if you're just overlanding, and not hard-core wheeling it, it can be fairly cheap to get where you want to go. I drove my first cruiser at 19 (SA spec'd '84 FJ60 in Peru back in 1987), so I get that the addiction starts early. Most of your money will be spent in gas, and then the basics to keep it on the road. All the petty things that go wrong, like headlights, oil (7+ quarts every change), minor repairs, etc. In not much of a particular order, here's my thoughts...
1. Tell us more about this history of the rig. Family owned? If not, how many owners? How much maintenance/repair history does it have? Where did you buy it? The answers to these will determine how much time/money/effort you put into what others are calling "baselining" it.
2. What's your mechanical aptitude? If you're into mountain biking, do you already wrench on your bike? If so, you can learn to wrench on the rig. Honestly, if you're not mechanically inclined, and have to rely on dealer/mechanic for fixes, these rigs will eat you alive financially. Repairs are infrequent on well-maintained rigs, but parts are costly - WAY above average prices. Remember, your rig sold for over $50K new almost 20 years ago. Visit the FAQ, there's a link for a downloadable FSM (Factory Service Manual). You'll want this. Your '99 will be slightly different in places, but if you need some section scans, just ask. I have a '00 FSM that's more like yours, but still with minor differences.
3. Do you have access to tools, jacks, jack stands, and a garage to do your own work? If so, you're in biz. If not, get another job to pay the mechanic. But seriously, there's lots of nice folks in the greater Phx area that would love to help you out/lend garage space if you're willing to put in the effort and learn. See #4...
4. Since your profile says you're in AZ, head on over the "clubhouse" sections and join up with Copper State Cruisers, some of the best guys and gals around. Go on a monthly run with them. They have many that are stocker friendly. My daughter drove Box Canyon by Florence with them when she still had her learners permit (with me in the passenger seat). They're patient with newbies, and a family-friendly group. My other daughters have all come on, and driven on, club runs. Don't worry that you're young. Bring a parent and come on out.
5. Since you're in AZ, get to know a user here named "murf", the service manager at Camelback Toyota at 16th street and Camelback in Phoenix. Don't bother telling us you live in Kingman, or Sierra Vista, it's still worth the drive... ;p. His guys KNOW cruisers, they work on TONS of them, of all varieties, and the CSC discount is worth it. I drive 25+ miles each way past 4 other closer dealerships for parts and service when it's something I can't (or don't have time to) tackle myself. Their website has decent prices on parts, and can ship if you live somewhere remote.
6. on to the cruiser... Don't let talk of engine/transmission rebuild scare you. The 2UZ-FE engine is one of the best Toyota's ever made. I don't think we've really found the end-of-the-useful-service-life/mileage-where-rebuild-is-necessary yet.
7. Ditch the brush guard. They're called "damage multipliers" for a reason. Especially if you're tight on funds, head on over to Camelback Toyota's website and price out some new headlight and grill assemblies and a front fender, then see if you can afford to keep it... (They're damage multipliers because they're not strong enough to ward off major impacts, and then they just bend back into parts and damage them when they otherwise might have missed the original impact.)
8. As far as maintenance goes, there's only a few things that will kill this truck. The biggest one is rare, but does happen not infrequently, and it's overheating. These engines run amazingly cool (especially compared to the 2F and 1FZ-FE engines in the 60 and 80 series) in our hellish summer temps. HOWEVER, there's 2 really cheap plastic OEM parts that can crack, and dump your coolant in a hurry. Start with replacing those, if not recently done. Search here for "heater T's". Many threads will pop up. You'll want a few new small hoses to make the job easier. SERIOUSLY - DON'T SKIP THIS, unless they've been done within 100K miles. I'd do it before I addressed things like tranny fluid or differential fluid replacement. Peruse this forum and see how many dead trucks there were because of old front diff fluid. Yeah, none.
9. Find out how old your radiator is. Mine cracked at just under 250K, and again, if undetected the overheat can kill your otherwise-indestructible engine. Timing belts can go significantly longer than the recommended 90K interval, but water pumps tend to go a bit sooner. I plan around 120K-ish for that service. When's the last time yours had it?
10. Then start going over the basics like oil change, flush coolant (just use the Toyota red - no need to cheap out on these fluids, I stick with OEM based on the longevity of them, not worth cheaping out here.). PS fluid, brake fluid as you can get to it.
11. Now, on to the important stuff, like how cool it looks. Lose the upper roof rack. Running it will make your abysmal MPG more abyss-like. Put it on when you need it. I know it doesn't look cool without it, but that's reality. And since your brush guard will be gone, you can loose the brush wires. We don't have brush here, we have hell-plants with spikes that will still scratch your rig regardless of the wires. But luckily, your rig is white, and they won't show. Thread over in Copper State Cruisers on getting out the Arizona pinstriping.
12. Lifting it - You can read threads until you're blue in the face here about lifting it, so I'll cut to the chase. Without a lot of new part$ (UCA, etc.) you don't really "lift" it, rather you change where in the normal suspension-travel range it rides due to the front torsion are suspension. Instead of having equal up- and down-travel, you have limited down-travel, and more up-travel. Make sense? You do this on a budget by buying rear coil spacers (~$50 from Slee or Man-a-Fre - prefer Slee due to lack of outrageous shipping charges) and put them on top of your rear springs. That lifts the rear 1 1/4". Then you crank your front torsion bars to match with a floor jack and a 30mm socket. That's it. You just "lifted" your truck.
13. If you've got the Cadillac-wallow, no need for expensive shocks. $130-ish and change from Camelback will get you all 4 corners of OEM shocks. They're fantastic for a stock rig on the road. They won't last forever off-road, but the price is right. I've blown a few of them, but they're cheap. Did I mention they're cheap???
13. Looks like you're already lost the running boards. You just gained some ground clearance. Don't do anything stupid without sliders though. Again, just peruse Camelback's website for rocker panels, doors, etc. and see if you can afford to try anything crazy. Sliders run ~$700-900, depending. Or you could just be careful...
14. There are several threads on here about what needs to be done to high-mileage cruisers. One has "200K overhaul" or something in the title. At that mileage, some of the bushings start to wear, and things get loose.
15. Speaking of loose, how's your steering? Does your rig go in random directions? Do you have a 6" dead spot in your steering wheel? The '98-'02 steering rack design is retarded. $50 in poly bushings solves A LOT of the problem.
16. Do you have any fluid leaks? Track 'em down and fix 'em.
17. There are a few killer-defects that are VERY rare, but pricey. In a '99, I'd say your biggest one would be the brake booster assembly failure. You'll know it when it happens.... Don't think there's much you can do to prevent it. Some folks have rebuilt, others just buy new at near $2K. At 400K miles, I'd just go new. Someone with a '98/'99 will chime in if I'm off the mark here. '99's are rarely affected by tranny failure. Don't sweat that either. Drain and fill tranny fluid on some kind of schedule.
18. Have you ever off roaded before? Are you familiar with how "lockers" operate? You have a center locking differential (all 100 series have them) and 99% of the '98's and '99's came with a rear differential lock. Read up on "how-stuff-works" on how these, uh... work. The biggest thing is DO NOT ENGAGE THEM ON PAVEMENT IN THE SCHOOL PARKING LOT. Or any other location with pavement. You'll end up with expensive repairs. Go out to Sycamore Creek and see how they work, and what a difference they can make.
19. If you're going to off-road in AZ, you'll want to air down those AT tires. Ask for some Staun Deflators (or similar) for Christmas to make your life easier. Then you'll need a budget way to inflate them again. Don't blow hundreds and hundreds of dollars on super-compressors or air tanks, just google a Q Industries MV-50. It'll last 10+ years of infrequent use. It'll air up all your tires in 20 minutes of so.
20. Make sure you're spare is in good shape and inflated.
21. Check your CV boots. The CV's in these are massive, and will last a long time, unless they start slinging grease out and run dry. Look at the bellow's-like boots on the shaft going from your front wheel into the front differential. If there's grease and road grime all over, time to hunt down the offending leak and fix or replace. Only use OEM CV's, aftermarket's tend to fail pretty quickly making them uneconomical. You can reboot/re-grease if you're willing to do the work and have the tools/space for cheap. New ones are around $400 ish plus labor, IIRC.
22. So for less than a few hundred buck, you can be up and running, confident, lifted, and out doing what's most important, actually USING your Land Cruiser for what it was intended.
Good luck, man!