Thanks guys, I think I'll go with both then.

For the moment I've just got a computerfan that's producing some air movement inside the cabinet. I don't have a fresh air input but the plants are small, the cabinet is temporary and I produce some co2 from time to time in the glass bottle so I guess it's allright.

If I leave the HPS I'll have to have a cold air fan blowing on the plants though cause it's really producing an incredible amount of heat, more than I thought it would.

Again thanks for the advise