I know this is of no use for this year. But it may be useful in the future. I happened to be surfing around and found a "HOW TO BUILD YOUR OWN R2D2" web page. While snooping around I found that they have a nifty little motor calculation spread sheet.
The interesting thing is that it takes into consideration, rolling resistance and grade (angle).
enjoy, just click on "Speadsheet to calculate motor requirements"
located mid-page...
http://www.robotbuilders.net/r2/build.htm