Nice....sounds like the same setup I have....pro w/servos and power drawbar. More spindle HP is my biggest "want" with this machine.
I can't tell what your load meter is reading on the PDB display. If I get mine up around 60%, there's a good chance the spindle will stall. I get a bit worried when it starts going above 40%. I've recently had it bouncing up over 50%, but as a rule, I don't tend to run continuous cuts over 30%. If it's a long tool path and I want to walk away, I make sure it doesn't stay over 30%. I'd rather the part take a bit longer than have the spindle stall. If the spindle stalls, it takes a second for the vfd to fault out. During that time, the servos will not even notice the increased load and will snap your tool and may even push the head out of tram. Here's an example. This is a programming error, but it shows just how easily the servos will snap a 3/8" tool
lol
https://www.youtube.com/watch?v=hfS35aGmP3I
I'm pretty sure this cut is about .500" doc, .030"woc and .003ipt on a 3F carbide running at 6000rpms
https://youtu.be/FxM7Hc3m1XA?t=253
I tend to run .003-.004ipt on a 3F carbide for roughing. Depending on the DOC, I will go .030-.060 for the WOC. I pretty much always run at 6000rpms in aluminum.
Oh.....here's a HUGE tip with mach3......don't stop the machine with the estop and never use feed hold if any of the axis' are moving. Use the ESC key on the key board. Everything just stops. The drivers maintain power so the Z axis won't drop(mine does because I still haven't replaced the gas assist shock).
Since it's been previously owned, you may want to go through the vfd settings and make sure they're all where they're supposed to be.
I'm planning on converting mine to the Acorn at some point.
Interesting spindle option you're looking at. I'd still want enough torque for tapping at low rpms.