Re:
Wouldn't touch an Audi from the past ten years or so, but having had Volvos for the past decade and more the parts are not particularly expensive at all (compared with similar class of car, BMW, Merc etc.)
I would say I've virtually always found the genuine Volvo parts, or their genuinely OEM equivalents, to be really top quality; you fit something from the blue box and you can be pretty confident that you'll not be doing the same thing six months later through material failure.
Both the older Audi-derived (850 era) and newer Volvo's own 5 cylinder diesels seem to reach massive mileages without much bother, so long as you keep changing the useless water pumps in the VAG engine.
I've never had a Merc, it'd be interesting to hear how you get on with it... the rust problems from the past 15 years or so really put me off, same with slightly newer BMWs - I quite fancied one for a change but think I'll be going for another Volvo if the current one doesn't survive the upcoming MOT.