Can Mesa do dual feedback?

None of those requirements are a problem for Linuxcnc. However setting up a tool changer does increase the difficulty of configuring the machine. Adding a tool changer adds a lot of logic that you must be able to work out.

Why do you think you need a servo for the spindle? (Linuxcnc can do rigid tapping with a conventional spindle motor.)

Logic isn't a problem, sounds fun (heh).

I priced VFDs + induction motors and servos at the same amount when doing my budget. Also, the BLDC servo is quite a bit lighter than the old single phase motor that came with the mill. I read it was necessary for rigid tapping to have a servo as well as being helpful in keeping the orientation of the tools in the changer constant when using one. Will this be a problem? I already have the servo.

Anyone got opinions on my other questions, e.g. which board to use? ethernet vs other? thanks.

It might be nessisary for rigid tapping on some other controls, but it is not required for Linuxcnc. Linuxcnc syncs the motion of the axis to the movement of the spindle. You can turn off the spindle and turn it by hand and Linuxcnc will follow the motion through the tap cycle. All you need is encoder feedback with index for the spindle position, and a way to spin it forwards and backwards at speeds where the axis can keep up with the threading motion.

I don't really all know the advantages or disadvantages of using a servo for a spindle motor, other than possibly being rather speed limited (most servos max out around 2-3000 rpm.)

I am also interested in using dual encoder feedback.

I purchased a 10k rpm brushless servo for my spindle motor also. It is about 1.2kw. I did so because of weight and motor speed.