aftermarket support is spendy if you go with MR2 "community" vendors.
tuning options are out there. the 5SFE would probably be better controlled with a GM ECM than a honda ECU for 2 reasons: 1. you'd have to rewire the harness to support sequential injection, and 2. the honda distributor is too small for the mounting hole in the 5S, but the 5S dizzy has the required teeth to drive an HEI module/GM ECM. that'll give you the ability to tune boost if you like. the stock 3SGTE turbo manifold bolts onto the 5S head as well. the S54 gearbox can handle a fair amount of torque as long as you don't try to launch the car. high torque can shear the teeth off the ring gears on hard launches. the E153 boxes in the post '92 turbos are mostly bulletproof. earlier E-series boxes has synchro issues that were solved later on. i have a 94 LSD box in my car.
It has leaky oil/water hose that can not be reach unless you got a chick-sized hand.
It has fuel pump that is ridiculously hidden.
It has a 2 inch gap for you to do the timing belt.
none of these are as bad as they sound. i have knuckledragger hands and i can do a timing belt in about 2 hours. same with the hose from hell (the one under the turbo manifold). once you remove the oil cooler, it's easy to get in there. the fuel tank isn't as bad as it sounds either. on my first try, it took about 45 minutes to drop. it helps to have someone working the filler hoses up top while you're under the car.
engine swaps are pretty easy in them as well, since 3 of the 4 mounts are on the gearbox, and many other toyota engines bolt up to the E153 and S54 boxes, including newer engines like the 2GRFE.
they're fun cars, but they're not supercars like so many MR2fags think they are. they can be built to be fast, but it all depends how much money you want to throw at it. if that car is >$1500, i'd pass.