In my opinion, there are minimal, but desirable differences between US and Euro M5s. I would like a 1995 M5, with a 3.8L and six speed box', but that is about $35,000 including federalization. I can get an absolute mint condition M5 here in the US for $25,000 (you just have to look around and know where to look email me if you want help with that). I would be willing to drop $35,000 for a pristine, <5,000mile US M5. And by choosing a US M5, I have saved myself a lot of hassles. If I want more power, there are ways of bumping up the hp, with out "taxing" the engine.
There are many more Euro M5s, and a lot fewer US M5s. I dont know how to accurately compare that (maybe an econ or business guy could do that), but I cannot. I think that if you wanted a Euro M5 produced after 1992, the difference would be dependent on what exactly you brought over. Here is a website (and a fellow M5er) that imports euro M5s:
http://www.willz.ca
That gives direct prices from Europe (I dont know how much he inflated them if at all), not including federalization which generally runs $10,000 and a lot of lost hairs on your head.
Nice Porsche. I would like a ~1991 911T, just to rip around town in. Of course some RUF mods would be installed....heh cool!