The die grinder as a spindle are great when the material you are cutting is can be cut at high speed (20krpm), but if you need to cut harder material, or use a larger cutter, you need to drop the rpm below 5k. These machines stall at low rpm.
I'm tinkering with using the motor either via pulley (2:1 ratio), or a "low" speed 5krpm 5kw direct drive of a spindle.
The example below is an inrunner, with a straight shank ER11 replacing the existing shaft. The end will be machined to hold 2 angular contact bearings.
This is for a cheap spindle (compared to $5k industrial spindle). I want to machine steel (1/4 carbide cutters at about 2-5krpm) and aluminium/plastic etc at 25krpm. I may need 2 spindles for that, but if this design works that will be cheap.
There are some examples on the net showing other designs like this.