M4 will have the S55 (twin turbo built N55).
If money weren't an issue, I'd have an Alpha 12 GTR, but if I'm looking to keep it under $100k... I might consider an E92 M3 with the stage 3 SC... should be good for 700whp and a lot of fun... plus you just can't beat that tuned V8 noise at 8,000 rpm.
The E9x M3 will be special forever... the only V8 powered M3 to ever exist...