Why not just buy a M5
Hammer mentioned something about lowering springs for stangs, which is a great improvement in the looks and handling.
Thats one thing about BMW's that I absolutely love, they sit PERFECT from the factory, the wheels are tucked up in the fenders nicely and the ground clearance is just enough to still have an aggressive look.
I'd stay away from the V6, but thats just me. By the time you spend the money to increase the power to anywhere near the GT's 260, you coulda bought a GT easily. I've seen used 99GT's with farily low mileage for right around $14,000, some even cheaper.
good luck, and welcome to the family