Drivers I'd say some Leadshine EM's (or AM's - they were superseded by the EM, but they're still available from some sources), as they're some of the best digital drivers you can currently get.
Hopefully somebody will give some other suggestions, as I don't really keep up with latest stepper driver options.

I just had a quick look at the wiring diagram, and I see the router uses a 45VAC transformer for the drives, so it should be producing around 64VDC, give or take a few volts.
And having just found a datasheet for the motors, they must have the coils connected in series, as they are quite low inductance (0.9mH), which means the theoretical max voltage should only be around 30V, fi they were connected in series.

Rewiring to parallel (if the motors have all 8 wires available), and changing to a lower voltage power supply could be an option, and most likely work out cheaper with better performance, as lower voltage drives are cheaper.

Personally, I wouldn't use a KStep for a router. Although they use pretty reasonable drivers, they're not as good as modern digital drivers. To me the KStep is ideal for something like a milling machine, where speed isn't really a top priority, or where you want a reasonable performing compact controller, not for something like a router where speed is a high priority. I do have one installed on a digitiser, and I think it's a brilliant compact setup, but that machine is not setup to be whizzing around at max speed.