Curiosity..
 

What are the known common things to look out for when buying an 04 STI? Also I’m curious as to what power the stock internals can deal with?

It would seem that one of the things to look for, even on a newage STI is rust, they seem to be going in exactly the same places as the classics, sills, wheel arches, rear strut towers, and the front c frame, saw photos of a hawkeye wheel arch the other day that had holes in it, try and avoid anything that has spent any time north of the border or on the coast, ideally find a nice clean Japanese import that has never seen a grain of salt.
Other than that it's the same as any car, lots of maintenance history, make sure its had regular, quality oil changes and cam belts (every 5 years unless someone has done high mileage). If I looked at one that had a dump valve and induction kit with no evidence of a remap I'd probably have concerns.

Thanks mate, It seems I hear a lot about headgaskets and gearboxes, is either a common weakness or just maintenance neglect like a lot of cars?

As shane said.

Headgaskets and gearboxs not really in the 04 sti,
6speed is strong just make sure sellects smooth at higher rpm, headgaskets are very uncomman if looked after and not messes around with.
Rust thats the killer

Yes I’ve heard the synchros appear common although I also read that most people tend to just live with the occasional notchy crunch at high rpm and maintain with a certain gear oil?

early cars can be problematic with poorly synchro's but seems poor maintenance / wrong oil / etc is as much to blame........

in general its only the later EJ25 cars that tend to suffer head gasket issues with the UK 2.0 STi in particular being very robust engine wise
as Benji mentions above rust is the killer