Digital drivers from Zap

[Jonathan]

Nema 34 will have slower acceleration since it's rotor is bigger, causing the moment of inertia to be greater, but clearly the high voltage driver compensates.

Having different drivers on each axis is fine and a good idea to cut cost, potentially.

I think you'd be fine with Nema 23 (3 or 4nm) motors on X and Y. It's a similar size to my machine which runs well with 3nm and PM752.

If not already you should consider driving the ballscrews via pulleys/timing belts to get the most out of the motors.

[Gary]

As said, higher inertia means they will not accelerate as fast, but you are converting a mill, so it is not as if you are going to be spinning then up to 1000 RPM anyway.
3Nm motors with 5mm lead ballscrews would be ok for the X and Y and a larger motor on the Z.
I have an X3 and this is a similar size, but i think my Z is heavier, i used 3Nm on the X and Y and an SY85STH80 on the Z with a 3:1 ratio for the Z and direct on the X and Y.
Gearing also increases resolution but at the same time means you need to spin the motor faster.