(copy this into google search) SST-E545-RCX-4-2-D pdf
Should give you the data sheet for the servo drive series as top result.
It actually shows they have 2 inputs on the drive for 24-90DC input by the looks of it.
If this is correct I reckon I'd go 70vdc with between 800-1000w in total.
I always go as close as safely possible to a drivers max voltage to guarantee good motor velocity.