Stepper Drive (like G540, but preferably USB like a Leadshine MX3660 or MX4660)
The Leadshine drives you mentioned are not USB, and are no different than a G540. Both will plug into a UC100.

Mach 3 or LinuxCNC? (Are these compatible with USB?)
The UC100 will only work with Mach3 or UCCNC. (Mach4 at some point as well)
I'd recommend UCCNC.

LinuxCNC only works with Mesa boards.