I've been debating on whether or not to open up the can of worms that usually comes with this kind of thread by replying, but what the heck, why not.

No one can tell you what's best for you. We don't really know your situation. I personally love the MKIII (86.5-92) but being involved with them for years, I wouldn't recommend them to the average Joe. To answer your question about plentiful aftermarket support...........the MKIV (93-98 ) Supra has tons of aftermarket parts available and makes stupid horsepower with choice parts. The MKIII still has plenty of parts around but I think it's safe to say that there's a bigger market in the MKIV.

Bottom line is that you're going to spend a lot of money to get a fast Supra. It will just be your choice to decide on the platform

MKIII ~ 1,500 - 9,000
That example being from a beaten up car to a low mileage clean car

MKIV Twin Turbo ~ 20,000- 70,000

Obviously these are ball park numbers and the higher priced may have lots of work into them already.