Will 04 and newer 18" rims fit on an 2002 A8L?

 

Yes. Pic here http://www.migalki.com/photo/pix/30211.jpg

And vice versa, D2 wheels will fit D3 A8s

 

size-wise, you can pretty much swap any Audi wheels onto any other Audi, as long as you keep the +1, +2, -1, -2 tire sizes in perspective. Well, any from about the mid-90's up to current. 5x112 +35 to +45 offset. something close to that. 7.5" for 17's, 8.5" for 19's, 16's or 18's adjusted accordingly.

 

even with the large s8 brembos in front? what about aftermarket rims from ebay and the like, do they need special hub rings or lug bolts?

 

depends on the offset... With the brembos I *think* they'd need a +42 to +48 offset, but I'm not 100% sure about that.