You'll need to vent the closet either way; the size of the exhaust fan will increase as your wattage goes up.
If you are renting, it is easy to go to a discount building supply or architectural salvage store and buy a cheap replacement closet door, whcih you can hack up full of holes and then replace with the original door when you move out.
If you want to cover that large an area with a MH400, get the GOOD bulb- I've been covering the same size space with a Hortilux Daylight with excellent results.
Mmm what else. If you are running your veg room on 24/0 there's no need to even keep the door totally shut- you can leave it cracked and mount a good sized fan to move air. But be careful, because the far corners of a closet are hard to ventilate right, and I used to have problems with suffocation in my old veg closet. You'll want to place your smaller fans strategically to avoid that.