Cutting/tool capacity of CNC-ed mini-Mill/Drill machines

This is a CAM question -- really.

For my next hexapod machine, I am looking for a better spindle than I have
now. Currently, I'm using a trim-router, 30K RPM, ~1/2 HP. For Aluminum,
this is too fast. For steel, it is WAY too fast, and underpowered.

I've looked at the head assemblies for Grizley/HF-style Chinese mill/drill
machines, including the plastic reduction gears and 1/2 HP DC motors.

What is the capacity of these heads to cut steel and aluminum? What
diameter mills? What feed rate? What cut depth?

It seems that CNC requires more reserve capacity than doing work manually,
since the computer has no "feel" for the process. It isn't listening to
the pitch of the motor, and compensating for a tool that may be dulling, or
one that is clogged with chips.

Several of you have done CNC conversions on machines with this style of
head. What is your experience?

Carl