Forgotten about the torque aspect - as I suspect that I am more likely to do smaller fiddly jobs rather than large straight cuts, acceleration is probably more important to me than ultimate rapid feed speed so 1610 makes a lot of sense.

Y rail position subject to revision at next level of detail - can't work out if it's better to put them on the face of the gantry rails and simplify alignment or on top/bottom as shown which is better geometrically (I think) but more fiddly to adjust/align. Actually, the whole alignment thing puzzles me a bit at present and I need to do a bit more digging around to see how it's done.

I guess that the rail size thing, especially on Z, is to give a bit more space between moving and fixed plates. I've sketched something out using 15mm rail, and starting with 20mm plates I reckon I could pocket moving plate to take the nut holder, carve a groove in fixed plate for clearance. and still have enough meat left for strength.

I take your point about bearing block spacing - any useful rule of thumb figures for what I should be aiming at? Current sketch has overall length (between outer edge of bearings on X) as 250mm, which seems reasonable given two ballscrew feed to avoid racking; Y is 150mm (bit on the small side? But as drawn, this could go up to 200mm or so on the top rail); Z is 275mm.

Thanks for constructive comments (and I take "unambitious" as a compliment in this case - I'm trying to come up with a good engineering compromise rather than overbuild as I usually do!)