Sorry didnt notice the label tag was dog legged through the lower one.

Good idea to fix the blocks and let the rails move up and down on the z axis. Are they 4 way equal loading? Thats how I'm going to do it on mine. I would allow for a least a 1/3 of the rail to be held in the blocks. So for a 250mm travel you would need 375mm rails.

If its just for sign making then do you need so much travel? Why dont you just use the stand idea but have different size collars to rise it up in increments. Some people have also used multiple mounting heights on the z axis plate to allow for the cutting tool, might be worth looking at.

Its hard designing things , might just order lots of bits and make it up as i go haha
Yes but isnt that the fun part.