I think that's a fairly decent design you've drawn.
I looked through your posts, I see you designed one with rack and pinion years ago....did you ever build anything?
You're going to be cutting wood, so you'll want some decent speed. I'd consider 20mm (or even 25mm) lead on the X and Y and 10mm on the Z. The 10mm on the Z is so the Z can keep up with the X and Y if you end up doing alot of 3d carving, but depending on how heavy your spindle is, you may need to counterbalance the Z with a pneumatic strut, or simply live with the fact that the Z may creep down and rest on the table when you power off the machine.
Is your gantry a single piece of 80x160 T-slot? You could bolt a 6" x 3" rectangular steel or aluminum tube on the back of it to add a bit more stiffness. You could even fill that tube with some form of epoxy if you wanted to.
The first machine I made had a cutting area 5' wide. I never once used a sheet of anything 5' wide. I'm assuming that you have something in mind that you want to do on this machine to make it 5' wide? If not, consider a cutting width of just over 4' and length of 6'.
The extra foot of length is to park your gantry at the end, making it easier to load and position your stock with the gantry out of the way. Also it can allow for a spot off the end of the main table if you want to add a rotary axis, or cut something long on it's end.
I've never used an Acorn board, but it looks pretty well thought out on their website, and has some good reviews. You'll want a board that allows you to autosquare your gantry during homing. The acorn board can do this, so can the plug and play electronics from AVID. From what I saw of the Acorn board on the centroid website (perhaps there is an expansion that I an unaware of?) it can only support 4 motors. Meaning that you would never be able to add a 4th axis to this design if that is something you wanted to do in the future.
I've never tried the Automation 4 Less mounts, but the Nema 34 SYK mounts I have are really nice.
When pricing out your motors and drivers, also have a look at the Teknic Clearpath SD servos (which do not require drivers).
For drivers, I'd stick with Leadshine or Gecko, some of the newer Trinamic offerings are quite tempting but I'm not sure if those would be appropriate for Nema 34's.
For motors, the Nema 34 Avid (cncrouterparts.com) sells @48V should do fine, but there are others you can find that may be a little less expensive and can offer a bit better performance. If you're going to do this, just make sure they are low inductance, around 2mH or less, and get a copy of the torque vs speed chart to compare it to the ones from Avid (it's on their website) at the max RPM you wish to use.
I wouldn't bother with "closed loop" steppers, for the extra money, the Clearpath SD are a better choice IMO. But regular steppers will also get the job done.