For 350 you don’t “need” pistons; fuel pump, downpipe, race cat and free flowing exhaust, and a remap on that. I would go easy on the ignition. Min 98 ron fuel.
400 stock sti ic is still ok, injectors, turbo and a 3 port boost controller, the pistons will be living on borrowed time, but if you keep knock under control it’s not impossible.
The question you can ask yourself: should I pay a ton of money up front and then feel a little more relaxed, or should I enjoy it for as long as it lasts and then pay a ton once the pistons give up. 😉