I wouldn't say that $100 is near-impossible, you're just going to have to look extra hard through classifieds ads, auction sites, etc., before you're going to find what you want. I think what Klondike was meaning to say is that finding a brand new system for $100 or less is next to impossible.

And while having a Metal Halide for vegetating, and a High Pressure Sodium for flowering is ideal, you can get away with just using one for both cycles. If you are only able to get your hands on one, I'd go with the HPS, as it'll get your plants through veg just fine (maybe with a little stretching), and has just the right color spectrum for flowering.

As for wattage, get the highest you can afford with what you have. If you can get your hands on a refurbished 400w system, you'd have ample lumens to grow a few very healthy plants. Just remember, though, that as you start upping the wattage, you'll need to start upping the amount of money you spend on adequate ventilation. This includes exhaust and intake. Just adding a fan inside a small closed room with an HID burning bright will, in essence, turn that small room into a convection oven. And your plants won't enjoy that very much at all. I don't have any experience with the lower-end 150w systems, so I won't offer any advice pretending that I do...but I would still imagine that some sort of ventilation will need to be in place for optimal results.