I don't think you'll have as much fun in the M5(s65) around the track as compared to a 335/135i(N54) or an M3(S54), It's bulky and not quite as nimble as a smaller three series. My honest feeling if you want to build it up and add more power is go with an E46 M3. You can't beat the price to get into it and there is a plethora of options for power adders for it. Another point to add here is that the M3 is a born track car, it was made for the twisties right out of the box.. I think the N54 is still limited for power adder options (big horsepower) but would still be a formidable track car. It would still need a little TLC though. When I drove one I felt like it was made for comfort more than performance. All of the aftermarket companies have excellent customer service and will take care of you if you have any issues.
So in conclusion if you want a purpose weekend/track car, go M3. You won't be disappointed.
Custom Turbo Build By Lab22
815whp @ 625wtq 27psi E98
Last edited by dallasberry; 01-09-2013 at 08:14 PM.