The question has to be: What do you want to do with it & what power?
I have a watercooled 3 phase 800W spindle with VFD. It's good for engraving & routing & light milling cuts in aluminium & brass. It was the one that came with my Chinese 6040 router/engraver. I plan on adding a 300W DC spindle to my cnc lathe as an engraving tool when using it as an X,Z,A machine.
The VFD I have interfered with my cnc PC – a plastic cased Dell with metal spray RFI shielding inside. I tried shielded cable & putting the VFD behind steel blocks, but still got occasional glitches. When that Dell PC died, I used a metal case PC and all the RFI problems magically disappeared, so be aware of this possibility.
As for runout etc, very difficult to disentangle spindle movement & machine deflection. The bearing life of the Chinese ones is supposed to be 600hrs @ 20000 rpm, better than most hand-held routers, but not sure how that would relate to your machine time.
I have used mine with 0.5mm dia cutters and also 16mm dia cutters. Never had a worry with that part of the CNC. Mostly used to cut wood, brass, acrylic & casting wax.
Regards,
Richard