You can certainly try your existing wheels. Having 20 up front doesn't really worry to much if your not riding though craters. As I mentioned in my first post, 24 in the back, with proper tensioning, might work as well.
I'm 240 and have a new set of wheels that were just too good of a deal as take offs on a new bike and they are 24 spokes. I haven't ridden them yet though.