OK, ditching any thoughts of pre-fab Z-axis for now - thanks Clive.
Been slogging through learning F360...whew! Finally got a first shot at the X-Axis base:

Click image for larger version.  Name: X-Axis Base v1.jpg  Views: 297  Size: 117.3 KB  ID: 22882

I said it was a 1st shot... Anyway, here is my thinking:
I can cut the cross braces on bandsaw and then use mill to true ends up and make them square and same length. Then, I have a really nice Strong Hand jig to clamp them at right angles to side beams. I was then thinking of mounting linear profile guide rail on sides.

How much of a shot do you think I have of getting the 2 side beams parallel and coplanar enough that I won't have binding? (If I am even explaining this well enough to begin with) I could mount one side rail carefully, and then rig up a temp gantry to use to get the other placed - then pin it and tack weld.

Also, I jumped the gun and ordered a set of these:
http://www.automation-overstock.com/...W01P63&cat=146

with plans on using them for this axis. Dumb question: is this X or Y?

Thanks,
Wallace