Apologies for the book but...
The question with the 100 Series is less "how many miles" and more "how much rust?" whether its 200,000 or 450,000 miles all the maintenance intervals and parts lifespans are the same. A 200,000 mile truck where all the suspension is original and pulleys haven't been replaced is not better than a 350,000 truck that was baselined at 300,000. Plus you are most likely going to be doing much of the work yourself or doing modifications and upgrades. These become 10x harder with rust. To me its much more about the deal that you get. For one, you're best off buying a 100 series off a suburban mom or some professional who just had this as their DD and doesn't really know anything about cars. Most of them don't know what these are worth, that's your best chance at getting a 100 series for sub 8K, anyone else will try to gouge you.
Frankly if you are looking at building it out then there's not much point in getting an expensive 100K mile truck when you are going to rip out all the ball joints, shocks, bushings, and springs anyways. Might as well get 250-300K mile truck with loose ball joints and saggy springs and soft shocks for cheap since you are just going to rip it out and upgrade it anyways.
Engine miles are almost irrelevant. The only way I have ever seen or heard of anyone killing a 100 series land cruiser engine is the heater T's busting and loosing all coolant and continuing to drive until the head gasket goes. I think I know of one person who had lower radiator hose issue and lost all their coolant as well, that's it. So as long as they've changed the oil, the transmission fluid is reddish, and the T doesn't fall apart when you touch it, the engine is probably fine just replace the T's the second you buy it. If it hasnt been done and the truck is >250K+ then just plan to replace all vacuum hoses, pcv valve, and spark plugs, gaskets, etc. You'll have a tune up and it will run like new and you'll have peace of mind. Clean throttle body and MAF while you're at it.
If you're getting a high mileage 100 Series LC and worried about it leaving you stranded then do the following:
A) Get AAA deluxe, its like $140 a year for peace of mind and pays for itself in one use
B) Take care of the 4 things that will cause a vehicle not to start 1) battery - keep a battery pack self jumper in the car 2) Replace the fuel pump and clean electrical connections if unknown age 3) replace alternator brushes if unknown age 4) replace or carry a spare starter if unknown age
C) CV/Front diff issues - have tools to be able to remove front drive shaft and flanges and you will be able to lock the center diff and keep going
D) Carry 1 Jug of Coolant, 1 Qt Oil, 1 QT ATF, 1x 4oz bottle of AT-205 leak stop, OBDII reader in the truck
Everything else is just about avoiding headaches but virtually none of means the truck is a time bomb. For instance, look for leaks around valve cover gasket, test the brake booster (car off, pump brakes 40x, turn key to on, time it and listen for strange noises if it stops in 30-40 seconds you're good). On test drive feel for slop in the steering, steering rack is a pain the butt to change out but if youre gonna have this for 4-5 years who cares itll be nice to have a new one in there and you'll know it was put in right and you can change all the bushing and fluid while you're at it. Slee has an informational page and OTRAM has a 100 Series Pre-Purchase inspection video that is very easy to follow. At some point past 200,000 miles old is just old and you're better off with more cash in your hand to put into baselining and know everything has been serviced and replaced.
Don't mind the book at all, nice to have all the info condensed. I think I have decided to go the route of buying one for ~$6-7k and then I'm putting the rest of my budget into baselining it and making sure everything that I think needs to be replaced gets replaced. I have experience working with older land cruisers and other cars in general and I since I'm going to own it for awhile I'm just gonna do the work myself. Definitely have had an easier time finding deals with ones that were obviously DDs. Thanks for the tips about not getting stranded as well!