If this is a budget HW, I would say the relays you want would be 100A plus to do wheelchair motors or something like that. Anything bigger, you need a real controller. Also, I wouldn't be very keen on tech checking a relay and microswitch operated robot, so much to go wrong. You can use them, but extra measures to assure failsafe would be recommended.

You will find, driving into the arena with a bang bang system is going to be tricky :P