7. For servo application, accurate position of rotar required, PMDC is used AKA "stepper motor". because of the difficulty of manufacturing very large magnets, I doubt if these are available for a greater power then a couple of kilowatts. BLDC is normally 3 phase so the rotar can only lock at 6 positions , so the shaft position is only know to be +- 60 degrees.
1. Big magnets are difficult to make, so no.
2. Brush are better as commutation losses lower.
3. Ferrite would only be used for specialized motors with a very high commutation speed.
4. Brushed motors smaller - no driver module needed.
5. Brushed are cheaper.
6.To future proof, buy only motors with a specified frame size ( NEMA)
Frank