That size cab is about perfect for a 400w. Adding some lighting on the longer sides might be a good idea but probably not necessary.

The height to hang it depends on the reflector/cooltube you use. Keeping it adjustable is very helpful, use chains. One way to see is by holding the top of your hand under the light. If it's uncomfortable at that height, it's too close for your plants.

You will need some kind of ventilation to bring in fresh air and get rid of old air, but there are many variables as to how much you need. Most people use an inline fan for exhaust and have a passive (no fan, just a hole) for an intake. There are formulas online for the size of fan you'll need. There's one formula here: http://boards.cannabis.com/indoor-gr...-progress.html

Check out some grow logs and see what others are using as a guideline. Just off the top of my head I'd recommend something around 250-300 cfm with a speed controller. But a lot depends on your ducting, carbon filter, cooltube and the temps outside the cab where the air is getting pulled from.