Hi George
Brush less motor and ER collet are good but not sure I follow the design (call your self an artist....:naughty:). Is that the ER collet shaft mounted parallel to the motor and the bearings are in the joining Grey bands? if so then there is quite a lot wrong.

The bearings need to be as close to the collet chuck as possible and the spacing and mounting type can be quite critical. running the belt like that will also cause problems. If I were you I would mount the motor up the other way so the pulley is driving at the opposite end to the cutting tool, Like nearly all spindles are, and lower the bearing mounts.

What speeds and diameter shaft are you looking at? as these are also quite critical. A low speed (less than 10000 rpm) 20mm shaft is not to difficult or expensive but larger diameters and higher speeds mean expensive bearings to reduce heat and lower the rotating mass, good balance is also needed and oil or water cooling at the extremes.

As usual I have probably got the wrong end of the stick, but hopefully there are some useful points