I was thinking of a 400w MH HID.

Does this make sense? Do I need more wattage? 1000w seems like overkill. Should I prefer HPS?

There is a bewildering arrays of reflectors and ballasts at prices from ~$200-$500. What should I be looking for?

How long per day should I run them (I just want to keep them alive through the winter as relatively dormant as possible).

I am not concerned about heat... if anything, the garage needs supplemental heat. I try to keep it 45-55F (my fish like that...)

Thanks.

I would suggest running for 10-12 hours. I suspect the temperature may get higher than you expect, but it depends how big and how well insulated the garage is.

Look for an electronic ballast if this is something you plan to do each winter, the initial cost is higher but it saves electricity (and probably gives you more flexibility of bulbs to use in the future). For a one-off winter maybe just get the cheapest ballast you can find because you probably won't make back the extra initial cost of an electronic ballast just running for three or four months. Any reflector that gets the majority of the light onto the plants is good enough. Simply placing the plants in the corner of two white walls will gain you as much light as the best reflector, but still make sure you have a reflector of some sort covering at least the top half of the bulb, there's no plants on the ceiling!