Mylar and foylon do the same job, they are used to surround the whole growing area so no light is wasted. Foylon is more robust and easier to use but much more expensive.

1000 watts should be good. I have a 400 watt metal halide (plus some daylight) that keeps my tropical hibiscus and allamanda growing well over the winter, but they do stretch a bit and need pruning in the spring. It doesn't bloom them much.

I assume your garage stays above freezing all the time? That would be a concern.

I bought a system that had a high quality ballast and a glass shield to protect the bulb from me (and me from the bulb!) It was the Agrosun Radiant system. I've been extremely pleased with it.

You may want to add a humidifier (not sure about the humidity needs of your specific plants) as the HID light dries out the air quite a lot. I would buy a cheap minimum- maximum thermometer/hydrometer (hygrometer) and see how it goes first. If you can enclose the area enough, with enough plants (especially if some of them are water plants in buckets of water), you may not need extra humidity.