Hi Paul,
It really depends what you think you would like to run. Right off the top of my head, you'd use one channel for steering, one for throttle, and one for shifting, so that's three channels. That leaves you three more, for anything you might like. Most sound units are tied into the throttle channel, so the engine sounds increase with the load or speed of the truck. You could use those extra channels for running a dump trailer, lights, and maybe a 5th wheel coupler servo. One thing is you can never seem to have enough channels for these types of trucks, so get the most you can afford.
If 6 channels is within your budget, then go for it. Or maybe check out Spektrum's 7 channel radios and see how much of a price increase it is. Might be worth it.