Cheap milling machine spindle - BT30-ER20, air powered draw bar + Tear Down!

[m_c]

Suitable BOBs do make life far easier, especially with the micro sub-d connectors used.

The 'Ethernet' control is more likely to be EtherCAT, CAN, or ModBus.

ModBus is generally too slow for typical CNC use. Even at high baud rates, you're looking at multiple millisecond refresh times for a single drive, which gets compounded when you add more drives.
CAN works at varying speeds, with continual data packets between all devices, and quite a lot of microcontrollers have it as an option.
EtherCAT typically uses a fixed 1kHz refresh rate (slower and faster optiosn exist, but 1kHz is the usual), with data frames passed through each slave (aka drive) with each drive extracting and adding it's own information.

I've got a set of EtherCAT closed loop steppers, as I really like the idea of it due to simple wiring (just standard CAT cables between drives) and each drive lets you add inputs/outputs, but the LinuxCNC implementation is buggy, and I'm not invested in it quite enough to use any of the Mach compatible controllers.
I might dust of a RPi and see if any of the bugs have been ironed out of the LinuxCNC implementation some time though, although I might just sell the drives to free up some shelf space.