all the aluminium i have used so far is ether 40/80 or 80/80 bosh compatible from kjn, i don't think changing the z will be to hard, i have some spare 40/80 i can use however the motor mount may have to go on the top like you have drawn which isnt so great as i have a hight restriction and need to keep it as low as posable so will have to see what i can come up with.

i dont think its going to be posable to do the y with one ball screw any were near the centre as the z will get in the way which mean needing two which would be a nice build but would cost something like a further £350 compared to about £60 ish that rack and pinion would be so will have to have a little think.

I think the max size bit would be 19mm but most i think are just 1/2" shafts so nothing crazy. foam likes to be cut quite quick other wise you get melting issues so you don't need big bits.