I'm starting a design for a medium size (aiming for 800x620 mm overall size) CNC router with its primary purpose for Aluminum but will do wood also. I've been reading these forums and with this type of machine people generally are going something which sits on a flat table, with dual powered x-axis. Please check out the pictures for my current design progress.

Goals:
- 800x620mm overall size, currently translates to ~520x460x100mm work area in the current design
- Primarily Aluminum, but some woods/plastics etc

Electronics:
- NEMA34 Motors on X/Y/Z
- DMA860H Chinese Motor Controllers
- 48V Power Supplies

Rails/Ball Screws:
- SFU20 Bearings on X/Y.
- SFU16 on Z-Axis.
- All 5mm pitch.
- All Dual HGH2XCA linear bearings.
- HGR25 on X and HGR20 on Y/Z on the Gantry

Motor:
- 2.2kW Air Cooled CN Spindle
- 2.2k VFD

Frame:
- 2pcs 80x120 T-Slot Side Walls
- 4pcs 80x80 Front, back and two evenly split in the middle
- Gantry is formed by 16-20mm Aluminum plate with two heavy walled 80x80 T-Slot.

Key Questions:
1. Most people recommend 10mm pitch, even for aluminum - should I rethink this choice of 5mm and go to 10mm pitch? Speed is not a goal.
2. If recommend 10mm, can you mix and match? I need to buy my x and z ball, but y is already brought. X = 10mm, y = 5mm, z = 5mm? Is something like this ok?
3. Most people are recommending the dual X-axis drive, with a flatbed (no moving parts underneath).
4. What is the most common milling cutter size for a hobbyist CNC router like this? What length (from the end of spindle) should I assume would be most common?
5. X-axis is driven under the machine as shown in the picture, will this work?
6. Where does the spindle commonly get mounted on the plate? And where should the spindle generally sit inside the spindle bracket?
7. I've already brought 3 x 48V power supplies for the motors/controllers. Will this be enough? Or should I be looking at ~72V?

Any help is greatly appreciated! I'm about to start a long journey so want to learn from your lessons at the start!