maybe look at a Haas UMC for some direction on design. they work well and seem to be similar to what you are looking for in movement. i'm not sure how important the horizonal configuration of the spindle is to your needs but the Haas might give you some design ideas. i would go with a square style linear rail opposed to the round rails. like the previous person said you are going to need support everywhere. i would think the rotary combo hanging out as shown will become a problem area pretty quickly. not to mention the weight factor of that part of the design when it comes to raising and lowering that end.