Help select Servo type, conversion from steppers.

[JAZZCNC]

Right now MACH3 is set up to 80 steps/mm (and depending on ballsrew lead that is 800-3200 steps/rev)

Ok I understand your thinking slightly better know but still don't think servo's are required.! Also not so sure why you'd want to go so slow as 50mm/min but sure you have a good reason.?

From the comment above I'm notsure you have the machine setup very well. For instance you say Z axis is using 6mm pitch and you are set to 80 steps/per.? This would mean the drives being set at 480 micro steps which is an odd number for micro stepping and I havent ever seen any drive with this setting.!

Higher micros stepping really helps smoothen out stepper motors and with good quality digital stepper drives then motor smoothness is excellent. Throw into this mix external motion control card and it gets even better as the smoother and higher pulse rate also helps smoothen motor action.

By the time you have bought servos and motion control card along with probably having to use some gearing ratio to account for faster spinning servo's you'll be well outside your $300 per axis max budget.
1 x $300 would nearly buy you 3 x digital drives and motion control card which will make big difference to machine performance if setup correctly.

Servo's don't always make for a better machine and there is much more involved to getting the best out of servos. Often the extra resolution etc is wasted due to rest of machine not matching up.!