Thanks for the advice and recommendations! With just a 400 Watt Metal Halide a cooltube and additional inline fans seems like a bit overkill to me (though it would be nice to have for sure).
The closet I am wanting to use for veg will not be air tight (it can pull cooler air from the room), and I was thinking I could vent it out into the flowering area easily enough by running a short duct through the wall.
I have two 600Watt HPS for flowering, with cooltubes, and a 500CFM fan to suck air through the carbon filter and blow air out of the room, and I am bringing fresh air in from the next room over, my flowering room will be in the same room as the closet I am wanting to use for veg.
Currently the closet is 72ºF during the day with 3 26 Watt CFLs running 24/7 (that I am using to sprout my seedlings). I am using a heating pad under my seedlings to stop them from getting too cool at night.
Being in Michigan could I get away with a small amount of ventilation for the vegging area (my flowering area will have very sufficient ventilation) through the winter months, and upgrade it a bit once I have a couple harvests in the books and a bit more money to go with?
Basically what happened is I blew my budget on everything else, and didn't consider the need to properly ventilate my vegging closet.