If you are using a trailer I wouldn't be too worried about wheels.
However, I would be worried about your gears.
I would suggest a 110BCD crank (llike a Sugino) or a MTB crank and somthing like 24/34/48 (more range than Shimano reccomend, but it should work ok, if your worried then a 24/34/46 will work fine).
Also think about maybe a 12-27 cassette, or if you want to change to say a Deore RD, then a 11-34.
These are mostly cheap items.