Everyone has a warranty until something breaks, lol.
If I had to suggest one mod, it would be an access port with a pro-tune for E30.
The cobb OTS tunes make the car drive so much better, even without the power gain ( or lack there of ), the other things they change, the car is just so much more responsive and better to drive. A pro-tune takes that even further.
After that? An intercooler.
Then maybe spend a lot of money collectively on a bunch of small things that make the car drive better like... Boombq shifter bracket bushings, shifter mount bushings, a short throw shifter, a dead pedal, a cobb or boomba engine mount.
After that... The car starts to get a little more expensive.... Cyborg/ATP/DHM turbo kit, downpipe, tuning. Its all pretty much laid out now, much power do you want? How much can you spend?
I will say that Meth injection is WAY WAY WAY under used on these cars. Its a car that runs off timing ( with a stock turbo), and is SUPER picky about charge air temps. Seems like a no-brainer that a product that cools the charge temp so greatly and has awesome octane boosting effect, would really be a popular mod.
Its not even really expensive for like $350-$400.