Wise and noble? Well, I can definitely say I'm not wise, nor noble. Well, maybe a wise ass. I can say I have experience, and maybe a few grams of information. Heh. With what you're wanting to do -- ten plants -- and get the best results from, you're going to need a kilowatt of power. With that, I think you should go with an HPS ballast -- fit to use a 1000w bulb --- and an MH conversion bulb for vegetative growth. The MH conversion bulb can be used in the HPS system, and it provides basically the same light as a Metal Halide bulb. This would give you the best results without needing multiple ballasts. A 1kw system with a good reflector will cover about a 5' x 5' space, easily.

To go along with this would be some decent ventilation -- you're going to need to keep the bulb vented if the area isn't already ventilated well. Don't get me wrong, if this attic is as large as you say it is and isn't insulated, you might be able to get away with not having a cool tube, maybe just substitute it for a power fan blowing directly at the bulb / reflector.

Another thing to consider is the odor control. How are you going to kill the stench of those plants when they're flowering -- or better yet, already in full bloom? People will say Ozone. I personally don't use it -- prefer carbon filtering coupled with ionizing. Safe and actually healthy for the plant and can be ran 24/7 with only bi-monthly maintenance.