Hi Wallace,

I would brace the underframe a bit with either some vertical pieces mid span, or diagonal across each side.

1.5kW spindles often come with ER11 collet (max 7 mm), although some are ER16 (max 10 mm). But my advice is go for 2.2 kW and ER20 (max 13 mm) as it will give you more options. The extra power will help with the torque at lower speeds. They are all 80mm diameter for the main body so shouldn't effect your design.

If the gantry will ride on those side members is there enough clearance in Z movement to the bed? Check they are going to be high enough.