Cost for me is a major consideration I'm afraid.
I would rather have a machine that needs adjusting from time to time,than no machine at all.
I looked at ball screws after your comment (the original machine actually had lead screws not ball so my fault there) and while on short runs theyre fine - it is suggested for 3.0m that at least 25mm screws would be needed- at £300 each just for the screw that's a bit out of my budget!
I am seriously gonna consider going with square linear rail though - even if it's 15mm - £120 for 2 rails with 4 bearings@ 1500mm long each side and then double rail on the X axis as well.