so what would I be looking at to go from an a4 to m6? Is it worth the trouble or just sell the car and get a new one?

 

It's definately a major project. You'll have to change pedals, console, all transmission/clutch pieces of course and maybe the crossmember. Then you'll need speedometer calibration. Don't forget the gears are 342's in an M6.

The only way I'd even consider it would be getting a donor car with everything intact.