George,
Some of Ross's goal is hinted at in post#1.

Ross,
I wonder if you need to break this down a bit. Maybe convert a big old mill to cnc for the lower speed, big roughing out work in Ali, and concentrate on making a high speed VFD controlled spindle (20,000 - 30,000 rpm) for the gentle stuff and the routing in wood.

Taking a punt, several of the Chinese spindle boast 'imported bearings' or similar, so maybe they suffered early on from the cheap ones. I'll stick my neck out then and say go for the £100 A/C combo, plus a cheaper ball bearing at the other end.

As for the drive, I think that even with the parallel mounting idea, unless they are out of balance by the same amount, and the oob is in the same place radially, you'll not improve things.