I'd stick with the 48v personally. You'd get better performance out of the motors. And don't use anything besides the G540 unless it's some G201x's or 203v's. (But there would be no point unless you were using bigger motors.) What you have listed there sounds really decent. Depending on your drive mechanism, you should easily get the speeds you're wanting. If you go to rack & pinion, I wouldn't be surprised if you could get over 600 ipm with that setup.
One more note: The power supply doesn't have to be 12.5 amps for those motors. But I'd still try to stick with 48v.