The mileage isn't an issue if you have someone knowledgeable to work on the car and can get the majority of parts at a decent price. The 1.8T's are largely bomb proof if you look after them but they need to be maintained at proper service intervals with items like cambelt, waterpump, oil, filters, etc. all replaced when they need to be. Yoru one main issue may be the Haldex, I can't remember if the Haldex on MkI TT's gave many issues but it will need to be looked after and can cost a lot to repair if it needs it.
I presume with you mentioning a clutch it is a manual and a 6 speed at that? A clutch may cost you a bit as it should be a DMF unit which means you need to replace the flywheel, clutch and pressure plate along with a few other bits and pieces. Unless you convert to SMF which is another story altogether...
To give you an idea, I had a 2005 Audi A4 Avant S-Line which had the BEX 140kw 1.8T in a longitudional layout with 6 speed manual gearbox. I replaced the cambelt, rollers, tensioners, idler pulleys and water pump at 120k which cost aroundR3k in 2014 using parts from Goldwagen and the work carried out by a good friend who works at a local VW dealership but does private work on weekends. The clutch change though, with a few odds and ends was around the R15k mark in 2015, again with parts from Goldwagen and the work carried out by the same friend.