No choice in the matter, I need 3 machines but only have space for 1.

Not sure what errors your talking about, with the raising bed I will be able to lower the Z travel to almost nothing 99% of the time.

I will be using servos all around or steppers with encoders, so hope fully I can keep the bed level, if not i've seen linear encodes relatively cheap on ebay.

I would like abit of advice on the bed though. Do you think just 4 rails and 4 screws will do it or would it be worth adding say 2 in the middle? I can't imagine the bed itself would deflect much, not when it's made from 100x100 box.