1610 screws are a good choice for X & Y with 1605 on Z. With a cutting area of 600x600 you could use just one screw for the Y put up the centre. Get the end machining to suit the machine it makes no difference to the supplier.

I would not buy any electronics until you have the frame built. Re the Spindle in post #1 The water cooled ones are very quiet and generally cost about £210 inc. the VFD that drives it.

Good luck with the build.