If you've got the money get a decent fan
and carbon filter from a hydro store, rather than messing around. A good ventilation set up keeps temperature, humidity and odor under control. Do worry about the smell.
For your size cab and a 400w HID a 4" fan shifting 150 m3/h should be fine. Use an online unit converter if the units get confusing. Passive intake is fine.
Exhaust either under the ground floor (if you have suspended floors) or just into the house/flat itself. Exhaust to outdoors would be ideal but you've got to wonder about what it's going to look like through an I/R camera
. Black/white sheeting to line the cab will stop the lamp itself showing up too much.
As stoneric says the light you linked is gonna get hot, best to have a light with the ballast separate so you can have it outside the cab.
Don't worry too much about SOG, plenty to think about getting some mj from seed to finished flowering.
6 plants should be fine. Makes things easier if you have plenty room though, maybe I'd go for 4.