Looks like Forza 5 will be including a R56 JCW in the game. Source - http://www.computerandvideogames.com/435168/new-forza-5-screenshots-released/
Forza 4 has an r56 jcw and a clubman jcw. If you get the 2000-2005 pack you can get an 02 mini. I find it frustrating bc I want a newer r53. Not a fan of the old style steering wheel.
Honestly having driven all the MINIs in Forza 4 and obviously owning one in real life, I'm extremely disappointed in how the driving dynamics were created in the game. They handle nothing like how they do in real life. Hopefully Forza 5 fixed this problem.
They don't start your car out with a stock setup. Meaning camber and toe are set to zero. Also, mind you it's a video game, depth perception and scale hasn't been mastered. Let alone you're using a controller, that's not all that realistic. Try to copy a sheet of your cars alignment and see how that works. My mini in forza performs much like my car except for having every single upgrade.