Yeah that space will work well for a 400w but height and heat will be your real challenge. The 400 will do very good for that space, but you will have to wait and save a few bucks and watch ebay for a small centrifugal fan and a charcoal filter combination.
These 3 things are a must have for growing
good MJ: For vegging plants, minimum 3000 lumens per square foot of grow space, and for flowering, minimum 5000 lumens per square foot of grow space. Lumens is the measure of light energy that is put out by the lighting. The 400w hps will give you more than the minimum and will grow beautifull dense buds that will be very tasty and potent.
With that low of a space, trying to remove the heat will be difficult going any other route than a centrifugal fan. The duct fans are worse than useless for this because they don't develop any vacuume to pull the air out. With the proper fan you won't need an intake fan as the exaust fan will create vacuume on your grow space and pull fresh air in through small intake holes. You will need to create a few intake holes using something like 2"black pvc pipe with 2 90degree elbows to allow air flow but no outside light.
Any time you grow MJ, odor is a serious issue unless you are way out away from others and don't have visitors that mind the odor. MJ is very very very odorous when growing during the flowering period. Bigger plants grown outside can be smelled upwards of a 1/4mile away when down-wind. A charcoal filter will eliminate this issue completely when used in conjunction with an enclosed room vented properly through the filter.
Trust me as I have experienced this and heard countless stories from people here that have done the sam thing. Don't start growing until you can fully set up this space correctly, even if you have to wait awhile. Getting ahead of yourself will cause you loss of money spent in a hurry, plenty of headaches and aggrivation, and possible legal issues. Once done right, you will be rewarded nicely, and you will be happy you waited.
In the mean time do some reading on growing, and on plant training methods as you will need to do significant training for growing in a small environment.