Basic maintenance - hoses, gaskets, everything that will age. Especially the stupid little rubber plug on the coolant hardline at the back of the exhaust manifold, that thing is responsible for more warped heads on our car than anything else except the head bolt torque. And it's a $3 part.
Then:
- Replace your entire exhaust, starting from the turbo back, including the elbow. I'd suggest a BIC DDP & exhaust, but pick what you like. Make sure it's all 3 inch.
- Buy a boost controller from Dr. Jonez - it's cheap and works well.
With your money level, I'd stop there. It's going to be plenty fast anyway. The next step I'd do is a MAFT Pro, 550cc injectors, an AFPR, and a wideband O2 sensor. And some tuning.
Then I'd buy a better intercooler and some hardpipes. And tune it again. By now you'll have a really fast car - as in, whip 'vettes, Vipers, and the occasional exotic - but it's likely going to be fragile as hell.
So, then you rebuild the engine for whatever power levels you want to throw at it, take one last shot at tuning it, and you're done.