My audi had a dual mass, I'm pretty sure its just for noise and ease of driving. The single mass will feel more touchy. The thing to keep in mind is that the "bushing" for lack of a better word, between the two seperate flywheel discs does wear out, and is also to why they are so expensive. I would get an aftermarket kit with a single mass, plus they include all the little odds and ends like the throw out bearing n stuff.
I have done the clutch package on an e36, and my b5 a4, and the bmw was easier. Just find a good diy article online and make sure you give yourself enough space under the car. Its not fun though without a lift, but doable.
1999.5 B5 A4: K04, GIAC software, milltek exhaust, koni/vogland suspension, hotchkis sways, etc...