for a 350 awhp you would be on the verge of bending a rod....its still doable but on a very conservative tune. what you can do is install the turbo "kit" and run low boost, after you build your wallet again you can do at least rods, pistons can handle ~500hp. What year A4 is it that you have? if its pre 99 than you have the AEB head cylinder which it has better air flow compared to the post 99 cylinder heads. Many people "upgrade" to the AEB cylinder head for better flow on a bigger turbo. know the list that would take you to your desired WHP.

gt3076r (there are plenty more turbos like bullseye, garret, etc...)
3" turbo back exhaust
tuning: Eurodyne or Unitronic or standalone
high flow turbo manifold
Injectors (440cc maybe higher)
clutch: very important, clutch masters make very good clutches.

and little things here and there. MAke sure the car is up to par in maintennace before you make more hp.

any questions dont hesitate to ask