No point in Servo on the Z. A stepper can crash tools at more than acceptable speed/feeds.
The reliability will decrease as you end up with 10+ wires instead of 3 or 4.
Consider, maybe, 3 phase hybrid steppers. More than up to the job. Nema 34, for sure, direct drive on the Z screw.
Using belt drive will certainly decrease positional repeatability due to the springy nature of the belt(s).
My Syil has the Y offset from center a bit, allowing to drill OVER the edge of the table. A definite advantage.
You need a hole in the tower to clear the Y screw to get reasonable travel.
Uncovering the X ways by 30mm each end is definitely OK.
Make sure you include an oiler system, from the start.
With lots of the way exposed on Y axis, you need to be able to grind the gibs every 200 hours.
Consider linear guides. That would be a much better solution.
At the end of the Y screw, a crude circlip is fine. My Y screw runs inside an aluminium torch housing. Yellow is pretty.
Z axis? Direct drive underneath, or on top of the column. Again, avoid a belt if you can.
Z axis really needs linear guides, as the overhung design will always have spring back/head nod.
Stability of Z column?. That is a REAL problem. Mine is attached to a BRICK wall at the top. Best thing I ever did.
Z gas strut? Don't over compensate. 2 struts, 20KG loading will be perfect. Make sure Z can travel ALL THE WAY to the top.
You can't have too much Z clearance for longish drills.
A 50-60mm thick machined plate under the column, to make room for Z motor is a good move.
Lower part of existing Z travel is never used, unless you rotate the head (which is a pain to get back to vertical again).
Consider being able to mount a small 3 jaw chuck. It makes a great CNC lathe, whether vertical or horizontal.
Spindle speed needs to be at least 3000. Consider pulleys to achieve this.
Direct drive on motors has a definite advantage. Use couplers, for sure.
Side/bending loads on stepper shafts will fatigue them, and they will break!!
Waste the space, and put a window in your enclosure.
I just hang heavy rubber mats, by magnets, from head.
No enclosure. How do you put something even slightly oversize on the table, let alone look behind it?
A big splash tray, with removable pieces.
These are my observations, after using my machine for over 2000 hours, at almost flat out speeds.
Preloaded ball screw nuts, for sure. Limit switches? I used 2 on Z. I can reference on lower one saving travel time to top one when referencing.
All opto slots, anywhere in the travel, that is convenient.
X and Y are under the table and are surprisingly reliable. My Y way just has e piece of rubber sheet held in place by magnets.
Magnets get covered in chips, but saves them going on the floor. Keep it simple.
I can work to 0.002" X and Y, and even better in Z.
The quill is the most useful part of the machine. Don't disable it, as some have. Why would you?
Add a small removable stepper to the quill drive, and you almost have something that can be used as CNC EDM with almost no mods.
The quill drive would need limit switches to make EDM very simple.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.