Sizing a power supply

Have a question regarding sizing a power supply for heavier duty stepper
systems. I've been running my MaxNC with Compumotor OEM650 drivers (3)
from a single power supply rated 40 volts at about 10 amps.
I'm going to be using these drivers on the Millport I'm buying, and want
to run the amps at their rated 7.5 amps.
I would like to know what amp capacity power supply I ought to
locate/build to handle the worst case load, all three axis running full
power, plus maybe just a bit of margin. Also, I really want to run up to
70 volts.
Anyone have any sources for a 70vdc PS in an appropriate amp capacity?
I really can't afford off-the-shelf new, but am not trying to do this
for $25 either.
I have a PS from a medical device from which I can pull 50vdc at 10
amps. Would this be better that the 40v setup to justify switching over
until I can locate a 70v PS? (it's got outputs for several volt/amp
ratings, and a ton of wiring I wouldn't need and would strip out)
Last question, would I really be better off building up separate
PS/driver, or can I run all three off a sinle PS just fine?

Thanks,

Jon Anderson