I recently bought a used Surly Crosscheck that's set up as a singlespeed (it's not the SS Crosscheck model). I'm thinking about changing over to a geared setup, which would mean at least a new rear wheel. The wheels on the bike now are Mavic Open Sport (32h). I use the bike for commuting and recreational riding, and plan to use 35-38 width tires.

Any recommendations as to whether I should look for a rear wheel only or spring for a set? Also, any recommendations on brands? I'm happy to buy used if possible; wheel budget would be in the $200-250 range.

Any suggestions would be much appreciated, not just about wheels, but gearing (1X9? 2X9? Internal gear hub?), as well as derailers, cassettes, shifters, etc. (I'm in Minneapolis, so not overly hilly.) Any chance of keeping this project to about $350 and still get stuff of decent quality? My mechanical skills and parts know-how are still quite basic, so any advice would be much appreciated.