Hi, I too am in the process of building my first CNC, very similar design to yours. Have received all the components except the aluminium as I am still fine-tuning the design. I too was struggling with direct or belt connection for the stepper motor and if it helps I have decided on belt for three reasons... Vibration from the motor is dissipated through the belt, I am tight for space so I can have the motor much lower with the belt system and if required I can change the drive ratio by changing the size of cogs.
Can I ask what thickness aluminium you are using for the frame around the Z-Axis. Most appear to be using 20mm but here in the UK 15mm is so much cheaper (or rather 20mm is so much more expensive!). I was very surprised at the weight of the spindle when it turned up (80mm water cooled 2.2kw) so now am wondering if 15mm will do?