Since the existing driver uses 37 volts (odd number) you don't want to go below that as then you wouldn't be able to run the machine at the original feedrates. I'd want to use a higher voltage to get better acceleration, particularly as those are Nema 34 motors which will be slow on that voltage.

If you're after a '1-box' solution I could make one for a similar price to the one from DIYCNC, except with the 4.2A, 50V drivers and just under 50V power supply (or the 70V drivers). Plus including whatever else you need, spindle control etc. PM me if your interested.

Why do you say holding torque would be more important than feedrate? The forces on the cutter / machine are tiny when it's not moving! Alternatives to the Kress are the 1.5kW and 2.2kW water cooled spindles on eBay. Not cheap, but you get what you pay for in my opinion ... see recent discussion in other threads.