well, on that type of budget you should get 2 12v cordless drill motors, they are very cheap, something like £10 each i use these and they are very good. for wheels a company called robochalange do blue wheels which screw right on to your drill motors.
for speed controlers get some 15a electronizes, dirt cheap, about £50-60 a pair (you will need 2) then get a 12v nimh battery for like £20 or 14.4 for a little more speed and a couple of sheets of HDPE preferably 10mm+ thick 8mm lowest.