Ultimately it's just a 1.8t engine so a pretty reliable engine, for serious power is worth doing rods and might as well do pistons while you're there.
A friend has a Stage 2 1.8t k03s Mk4 which he's owned for years with over 160k on it and the engine is healthy and it does shift.
That car OP linked to seems to have a lot of history which is a good thing..