I agree that if you want to make the best use of your available space, you'll seriously want to consider a 1000w HID. I just started using a 1000w digital MH/HPS system in an area slightly smaller than yours, and so far, I'm really enjoying it. Heat is not an issue, but I've also got good ventilation.

You can find a lighting system like mine for less than $300, ventilation and odor control can be constructed for very little if you can do the work yourself, and like Pharma had mentioned, you could easily upgrade to another light later and just have to minor modifications to your ventilation.

And the height of your area shouldn't be a limiting factor. You can use a screen, or stress train your WW's and keep them at a manageable height. Mine rarely got over 4 1/2" tall, and had great yields per square foot of space used. In fact, many of the buds needed to be supported because even the reinforced stem couldn't handle the weight.