Dimmer switch will be fine if these lamps operate at 240 volt. 5 X 20watt = 100 watts so get a dimmer rated for about 250 watts. Buzzing is caused sometimes from the electronic circuitry within the dimmer, the more you dim typically the buzzier they get. But buy a good quality one and everything will be fine.
If low voltage because its got a built in transformer then you will have to check the technical data to see if the transformer will work with a dimmer. Some are fine to use with a dimmer.