Hinjeniero:
I know and that's why I need to know if enabling PWM will be as easy as BJTransistor + MOSFET,
It should be that simple, yes...
The amperage of those LEDs isn't too bad (300mA), that should make the PWM switching easier on the PSU than low-voltage/high-current LEDs. If you want to be extra sure you could add some extra capacitors on the power lines near the LED. Get come big ceramics.
Hinjeniero:
also there is not too much space inside of a ceiling spot light and I will have problems to dissipate 50w per led. Hope that running the LEDs at <55% plus a bunch of these fans the temperature won't raise too much.
Fans will make a big difference, yes
(if the noise doesn't bother you...)