I'm only going to use a 150 w or 25o w HPS light for my entire grow. Due to lack of "financial resources" Im not sure if I can afford a hood and ballast. I only plan on growing one or two plants.
The ballast fires the bulb. The hood distributes the light and allows you to contain the heat to be extracted through the use of ducting.
Yeah if you're getting HPS or MH, they work actually by causing a spark to jump across a gap in the bulb, not by heating a fillament like a regular old-style bulb. So to manage and control that process, you need a big transformer called a ballast, which is similar to the box on your laptop power cable, it has to regulate the electricity carefully so it does't fry your laptop. So if you have a HPS or MH bulb, a ballast is absolutely essential for it to work. As for a reflector hood, they include the right socket for the bulb, and they improve the efficiency of the light significantly, so it is certainly recommended to have something rather than just a bare bulb. Otherwise half of the light goes up, uselessly away from the plants.
You can get the smaller mh/hps setups with a built in ballast. Ballast and reflector are a must. There are some really reasonable setups on Amazon. Good luck