Dave I'd stick with belt drive on Linear bearings and just protect it good. V-pulleys are a pain in the arse on routers and with Plasma dust etc I'd think they would be a just has big a mare.

Belt with give you best of all worlds. Higher resolution, Higher efficiency, less maintenance, Smoother action. R&P is ok and works but more to building it to do properly and it becomes expensive. Belts will be cheaper and easier to replace if damaged.!

If was using a Ballscrew (which I probably wouldn't) I'd put it down the middle and run it thru a tube tunnel with slit in bottom. I'd use profiled linear bearings on underside like you have shown.!