If your looking for a low-cost solution to those requirements I would recommend getting the base of an old electric wheelchair. Lots have two rear wheels independently driven and casters in front, they normally have a 300watt motor on each side, have decent sized pneumatic wheels and have electromagnetic brakes( if this is needed for you).
The electronics system is pretty simple, and you might be able to use the original control system but with just two servos controlling the joystick of the wheelchair. Colin Furze did something like that and it a quite good cheap solution.
https://www.youtube.com/watch?v=nUpzC1QP8z4
Wheelchair motors are generally 24volts, and if you wanted to use other motor controllers, REV Sparks are probably the best low-cost version and they can operate on up to 30volts. The Talon SR, SRX and Jaguar are also capable for operating on 24volts.
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