I used 8' long T8 color temp 6500 fluorescent bulbs from Grainger's Supply, the higher color temp, the better.

400W sounds like about the right level. I recommend HPS for supplementing natural light. You may well find that at 45F-55F the plants are sufficiently dormant that you don't need supplementary light because they aren't growing. Under 1000W you might find things actively growing which could cause you problems.

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!