Try to borrow a 700c wheel to check it out before wheel building.

You could always run 700c up front and 26" in the back if push comes to shove. The back might sit a little low with small slicks back there, so watch your crank length. Conti 1.3, Ritchey 1.4, something like that. There's a Ritchey 1.0", but I think you need to have a pretty narrow rear rim to run that thing. 1.3 or 1.4 would be a good match for 28mm up front, assuming your fork/brake will fit a 28. Edit: actually I'd probably go smaller up front, run 23s to help reduce the overall diameter difference.