MK4 GTI's can take 300hp daily, APR turbo kits have seen 50k miles on stock internals putting down those numbers with stock like drivability.
But like everyone says you make any more than 300whp in these cars mods start to get exspensive. after 300-350, your gonna need to get a clutch , forged internals and ect.. Not too reliable IMO.
I'm on my 3rd trans.......
I would highly suggest a C5 or a C5 zo6. Buy it cheap drop in a slightly built motor that can handle spray and slightly trans and walk people all day. And I'm sure you can do that with 30k easily.
I got to drive Sams setup one day and to say the least it was a bad mother fucker. Shit if I had it Id drive it everyday.
C5 FTW TrueStory.com
My next car will be a Fastback or a C5.