Markos, that depends how the car was maintained and driven for those miles, it might be fine for another 50k, or it might be about to fail like my S3 one recently did at 106k.
Swapping to a K04 isnt just a case of buy the turbo and bolt on though. You would need a Manifold, downpipe, FMIC (or at least twin sidemounts like the s3), injectors,3" MAF, S3 airbox or jetex cone filter, Tip, various other pipes, and a map to suite, also brakes to stop it, as it will have 90bhp+ more than yours were designed for.
Also the intake manifold is different on an S3, so you may need one of these too.
S3LOL - The suspension on an S3 is a bit stiffer and lower as well so this would need to be addressed so you don't end up on your roof.
looking at the cost of upgrading, I would advise just buy an S3 in the first place. Unelss you have a donor S3 perhaps accident damaged to take all the parts from, its not really worth it.