Are you looking to have the wheels contained within the robot or are you gonna make wheel guards for it? (Im assuming this is for the metal chassis )

If youre going for the wheel guard approach, then Id agree with the diagonal drive system mentioned with the drills. If youre wanting the wheels contained then possibly the only option due to the height of the chassis would be zeobot motors (then again, theyre ~£40 each rather than £40 for two).

width of around 200mmm

Is that 200 milli millimetres then