Quick newbie question, 400 or 600 to start

I've got an area 5ft x 3ft sq growing area, just planning the setup, gonna go with DIY DWC hydro four pots.

im gonna use a hood with a glass cover and a heat extractor unit, digital dimmable ballast. Just not sure if I need 400w bulbs or go straight for the 600w ones, the guy in the hydro shop reckoned to start with 400w, just wanted to see what people on here thought.

And I presume I can set the light fan and filter like this? Or do I need two lots of extraction, one for smell and one for heat from light?