What do I need to look at specifically, looks sweet outside, interior is honest and engine looks unmolested
you know the usual mechanical checks. golf specific, rear brakes are prone to seizing, gearboxes are a weakpoint (i am on my third - they tend to try and throw the diff out the box), crunch when going into second.
if it is big money check all the functions work on the right hand stalk - outside and oil temp, mpg etc.(the mpg ones NEVER work lol)
what year is it? there were a few very early k jet 16 valve (look for xr3i style metering head) these are pretty robist. the more normal 16v will be digifant and ecu controlled. the ecu's are easily damaged by poor engine earthing - this usually results in the injectors being open all the time, as soon as the fuel pump primes resulting in a cylinder full of fuel or at the very least poor running.
rust areas - on the bulkhead behind the downpipe - difficult to check but look for damp carpets, all around the sunroof internals, inner sills , outer if a post 90 model with sill covers, around the fuel filler area., corners of the rear panel under the rear lights - there is a body seam that rusts and is an expensive fix. make sure this seam is there and hasnt been filled over