Riot shields to a wheel chair...? XD
While making one of the PMV's from 4th edition's Arsenal into a combat rig would totally work and be viable, if your character is physically crippled and good with tech, I don't see why you don't just get him a drone he can use instead of going in himself? You don't need to be a hard-core rigger to make good use of a single MCT-Nissan Roto-Drone. Buy one, invest a few points in the appropriate Pilot skill, a few in Gunnery, and mount a shotgun on the thing. It won't be a walking tank, but by making use of its sensors to see, hear, and possibly even smell out of it, you're prettymuch "there" with your group even without needing to be jumped in.
It's considerably more vulnerable than suping up a vehicle, but at the same time, it's much cheaper, and doesn't require the crippled man to get into a gunfight in order to contribute to combat. Keep in mind that it even going with the "wheel chair tank" idea would still require him to learn at least Gunnery (assuming he has Pilot (Groundcraft) already). If you didn't wanna learn a new pilot skill, there's a few other drones you could use instead.