depending on how much you have, a basic robot can be very simple. i had access to a old kit bot frame and kop trannys, took me almost 1 hour to totally hook it up, i didn't use a frc control system because i don't have one, the entire bot cost me zero, but its just a basic kit bot, everything was supplied in the kit of parts.
now im in to dewalt motors, try eBay, for like 20 you can snag a working motor and transmission, you'll still need to use a lathe to make a part to use the shaft, but its cheap and easy.
welding a chassis in really nice, and custom. but not entirely vital, depending on how big the robot is, you might get away with 2x4's arranged in a box, with bearings mounted to the wood, i used 2x6's and some plastic helical gearboxes from the kit a while ago, and it really turned out nice,(well as good as wood gets) it had pneumatic wheels and was extremely simple and adaptable.
http://i162.photobucket.com/albums/t...8/DSC05492.jpg
Good luck with getting something rolling!