I'd think about going with Discount Hydroponics.

Discount Hydroponics

They're price competitive with ebay companies, and you can get a switchable ballast 250w with an HPS bulb and an extra MH bulb for around $160. They've been around for a while, have brick and mortar stores, and have a good overall rep. The switchable ballast is worth it, IMO...spending an extra $30 for an MH bulb for veg to go with the HPS for flowering is a small investment that's worth it. My .02.

250w systems don't give off a lot of heat...I've gotten by with a circulating fan aimed into a grow area. You'll need some sort of air circulation. You also might want to watch the ambient temps if you're in a garage--especially in summer. Depends on where you live.