I think the cab is the right size for a 250 watt but a bit to small for a 400 watt. So a cooltube and basically whatever size light you go with, you should design the size around that. Have you thought of a grow tent or is this strictly a stealth setup? I recommend the hydro hut if your going to go for a tent.
It would work nicely with a 250w or 270w HPS....you can cut the two smaller shelves on the right move the divider wall (center wall) in the cab over a little more to make the left side (non shelved side) your flower area and make the right shelved area a mom/clone chamber with the help of some CFL's.
I'd use a 4" 172cfm or a 6" 449cfm inline centrifugal exhuast fan (can fan,vortex,eco plus,elicient) for exhuast and cooling the light.
250W HPS is better for that cabinet. You'll have 100w/ft2. If you grow seeds 12/12 from the beginning you can probably get 3, maybe 4 plants in there. You need to keep your plants low with an even canopy. If they get too tall, that 250 won't penetrate. Doing 12/12 from seed will keep plants shortter and even. Check the threads going now on 12/12 from seed.