Hi Mayo,
It's the same situation here with the "Indoor Gardening" shops. They cater mostly to the marijuana growers, with a focus on hydroponics, but they're also the only place to get the more esoteric gardening supplies. The problem with that whole business is that most of their customers have little to no gardening experience, and fall prey to the abundant gardening myth attached to pot growing. There's a point of diminishing returns when it comes to any horticulture, so while some of the more exotic growing techniques and advice do have some beneficial effect on the plants, it may come at a very high cost. The first $100 you spend on fertilizer and soil mixes will produce very noticeable and worthwhile returns, but if you spent $200 on "boutique" products, the result might be 5% better. Maybe.
With 50 6" containers, you'll need an area about 3.5 to 4 foot square, which would probably be most conveniently lit with the 400 W bulb and a proper reflector, plus side-reflection boards. I'm a little warmer here than you, and I think the coldest I ever found the garage was just under 40° F with outside temps of around 30° - assuming bright days in between to re-warm the garage and its contents.
I think a tent might hold more heat than you need after that light burns for a few hours, but some low-wattage heating mats under the pots would keep the soil temps at a good growing temperature. (I'm guessing here, but 55°F might be good enough—it'd be best to research this one.) Use a good thermostat with a probe you can stick in one of the pots, and you might also want to put it on a timer to come on about an hour or two before the lights. I'm not aware of any heat mats in that size, but surely someone makes a 2' x 4' model. If you're worried about reaching freezing temps inside the garage, you can run the heat mats around the clock controlled by the thermostat. If you provide long enough day length for the plants, I doubt many will try to go dormant. Any that do would probably be better off outside.
Put down some styrofoam insulation, then the heat mats, then a sheet of heavy plastic with the edges lifted up over a simple square frame of 3-4" boards to hold any watering runoff. You'll probably have to work to control spider mites since it will be a dry atmosphere. A fine spray of water directed upward at the underside of the leaves once a week will go a long way toward keeping the mites down.
If you don't go with a heat mat, then you might have to use a tent, just keep an eye on the internal temps. A small oscillating fan is always good to blow the leaves around to promote more sturdy growth and keep the temperatures even (run fans on the light timer). A serviceable tent is fairly easy to make with some plastic sheets hung from wood rails anchored to the ceiling. I don't think you'll need full enclosure, but if conditions force you to put a lid on it, make it high above the light so there's no chance of it catching fire.
It's only critical with the big 1000W lights, but nevertheless, make sure your electrical system is up to the task. You might want to run your heat mats on a different circuit than the light. Make sure the wiring in any extension cord you use for the lamp is rated for a continuous load of 500W. 14 gauge is good.
Sounds like a fun project. Take lots of pictures, particularly "before and after" pictures of the plants.
Ken
