It depends on how deep you want to get in to it. You could just put some Bridgestone BT-45s on your stock rims and hit the track. Most commonly guys will lace up either 17" (2.5 front and 3.5 rear) or 12" (2.15 front and 2.5 rear) rims. The stock suspension will probably be way too soft and the brakes won't stop it, either. My advice is to try to find something that is already built, you will spend a lot less money that way. Here's an example; a buddy of mine just picked up a CR85 set up with 17" Sun rims, big front brake and an Ohlins shock for $1,400. You couldn't buy the wheel set and shock for that amount of money.How or what did you do to the 105 to make it an SM? Just tires? I'm looking into Mini GP but in larger strokes. The 105 doesn't seem like a bad choice.