First of all thank you for the answers...
But I'm a little bit confused.
I know DC asynchronous motors and I know that speed is related directly to frequency, so I need an inverter to pilot them and this solution is not cheap for 220V motors. (as far as I know)
I know that DC motors are cheap but speed is not stable.
So I didn't understand which is better (price/quality) solution.
Maybe if you have links to shops that sell these motors I can ask directly to them.
Cheers, RA