My suggestion would be to get at 16t cog to fit on the cassette, and line it up with the outer ring on the front. 42/16 is a very common, relatively easy gear for riding on the road. Any chain tensioner would work, but try it without one first. You might be surprised. Lastly on that wheel... its your choice whether to use it or not. I'd say find a used wheel. If you're feeling like spending some money on it, buy the IRO wheel that has a fixed/fixed hub. It'll take bmx freewheels also.