I'd put in the v10 from the m5.
It would have TWO transmissions, one 6-speed manual and one soft 6-speed automatic both installed so you can have more options while driving.
There would also be a button (a switch would be OK too, but a button would be better) that you could press that would kill 6 cyl and all the advanced trickery so that car would get 30/40mpg. Scratch that, it would just get 30/40 all the time!
The seats would be rigid, strong, racing seats but they'd also be big and leather and comfortable at the same time.
There would be a dial that could change the exterior color of the car.
The redline would be 10,000rpm and it would maintain power all the way to the top.
There would also be a button (again, or a switch) that would make the car go super duper ultra fast.
It would only weigh only 2900lbs and be under US$50k.