7 speed specific is a dead-end. Go to 8/9/10. That way you can use the wheel on something else later, or upgrade to a different bike etc.

Even if you start out with 8 speed, you still have the option for 9/10. Then again if you never envision upgrading then it might be more cost effective to pick up a cheap 7 speed rear wheel. Those can be had for as low as $40.