Shane,
First, get the appropriate insurance coverage. If you're at that point of 20K you may want to consider collectors car insurance, you can declare a value for your car and then its covered based on your perceived value not NADA or KBB value like normal insurance. Long story made short I had a really good friend who had a Mazda RX-7 (FD) who had it COMPLETELY built (close to $60k in it), someone ended up wrecking it, and he luckily had a declared value of insurance on it and it was completely covered. So that totally saved his... uh butt..
The diesel conversion, like everything, really depends. Since you're OCD (like my wife and a good friend of mine [who builds subaru's and replaces EVERY bolt and nut everytime he takes them off]) Its like asking how much is a good bottle or whisky or a nice house. It depends on your personal standards. That is the singular best answer. If you want a brand new completely rebuilt everything you could easily put yourself into the 20k range. If you're okay with a red-neck do-it-yourself build, you're talking whatever the price of the donor rig is (really at that point you should really just oops switch VIN numbers)

Honestly 10k wouldnt be a bad estimate, maybe 12-15 if you want to keep it all toyota. I think the "2fe" conversion runs 1k-3k depending on parts.
So that all being said, here's my 2 cents for whatever its worth. I thought about the conversions and swaps every which way for a year until i was blue in the face. I am by no means a "purist" (trust me I wanted to put a gigantic v8 in my original rx-7 for years). I think there is a lot to be said for the original motor configuration. Do I think the emissions system should go as fast as possible? Yes. My LC went from 12 mpg to 16mpg just by rebuilding the carb, a good tuning and a desmog (god bless Missouri's emission rules). That being said, is it great for modern highways? no. Is it comparable to a new off road anything? No, of course not, its 30 year old technology that was designed for reliability not on road comfort! So to answer your parts availability, I will agree, parts can be hard to come by. But only if you don't know where to look. Sure 60's dont have as big a following as jeeps or 4runners, international scouts... well hell anything really. But the community is good and they are all passionate about their 'cruisers. That's actually the thing that made me want to open my business, I realized that no one really specializes in 60's, there's tons of support for 40's 80's and what so have you but 60's get the stick. In my opinion (for whatever its worth) its the perfect compromise between the no-frills 40 and the oh-gee-is-that-another-camry?-fj80. So I guess what I'm saying is what is your passion about the 60? Is it the body and the looks (cuz trust me it isn't the seat comfort

) or is it the off-road capabilities? If its the looks, and you want it to run like a modern truck you should just do a vortec swap and be done with it. If its the off-road awesomeness you should get the 2f all straight and get your rig beefed up. For me, what I love about my 60 is pretty much summed up in its name "the blue donkey." Sure it isn't fast, that's because it isn't a race horse. It isn't efficient, because it's shaped like a brick and weighs as much as 100 suns! But, it does two super important things, it makes me smile, and gets me where I need to go. When I first started driving it I had no idea that the entire 4wd was basically shot, but it still worked. What other vehicle works when it is missing 75% of its seals? The answer is NONE. My old Honda was sidelined for weeks because of a crankshaft position sensor. The 2f, its like "what the f is a crankshaft?" lol.
Anyways to end my ramble, there are a million ways to spend money on a cruiser, but there is only one way to enjoy it; and that is completely up to you.
-Ryan
Ps: feel free to pm me or call me if you have any questions, no time talking about 60's is wasted time.
PPS: hope this helped.

PPPS: I just realized all the prices I threw at you are what it was going to cost me to do it myself. So I apologize. It would all be 1 million dollars (sarcasm)