It depends on how much prototyping and sourcing you're willing to do.
Hot rodded versions of all of the 80's and 90's BMW sedans are well documented but you won't be as original as an Audi. While I like the idea of a hot rodded M-B, the engine bay doesn't bling as well as the BMW versions.
Here's a benchmark for E28
http://www.mye28.com/viewtopic.php?t=108450