I'd not be keen on it - at the 92% efficiency at 1kW (wet finger approximation to your 70V x 16A), that's 160W it needs to dissipate - the heatsink looks a bit weedy for that, so immediately I'm worrying about the duty cycle / overheating of the regulator. £20, though - might be an interesting experiment.

I'm curious that you claim you require 16A, I'm assuming 2-phase motors, so 2x5.6=11.2A/motor(x3)?, so >30A requirement, or is my understanding wrong? (I thought steppers excite both phases).