We have been racing OTK products for the last 3-4 years. I would suggest sticking with OTK for most 2 stroke applications. If your going shifter Sodi is another option. Right now everyone wants a 206 or KA. If you want to go fast and are not concerned with a lot of racing competition at the club level, used Tag engines are very reasonable right now. Just a little higher than a new built 206 ($1200 - $1500).
Again if you just want to go fast, a good option is Rotax. They have long rebuild cycles and are relatively in-expensive to operate(more like 10 - 30 hour rebuild cycle). X30's are reasonable now, but need rebuilt every 7-10 hours. A top end rebuild is $800 - $1000. A bottom end is $1000 - $1300.
Personally, I don't get DD2's. A well driven Rotax Max is almost as fast, much more common, and doesn't have the unique issues a DD2 has.
OTK parts are available locally at Crosslink or online at several paces. OTK tuning information is widely available.