much easier to get mtn/hybrid wheel in first place.

http://www.amazon.com/Avenir-Shimano...ef=pd_sbs_sg_6

you'll probably need a spacer to put a 7 speed on there. for extra fun, the derailleur MIGHT need readjusting to swap wheels, but maybe with the right spacer it won't. or, put an 8 speed on it and just use the taller 7 gears, this IS for a trailer, after all.