powered woofer

After considering the options for buying/building a subwoofer, I think I've decided that what I would really like to build is a powered woofer. I don't need it to go lower than 30hz maybe not even lower than 40hz, but I would like it to be good for music.

I really don't know where to start with this, but I am imagining a small powered box with a 8" woofer that would just fill in the missing lows from a set of small speakers (possibly Orbs, but I have not decided). I don't want to spend too much, maybe around $150.

Questions:

1: What do you think, does this make sense?

2: Where should I start to think about designing this?

3: what specs are most important in a driver for this application?

4: is there a big difference between woofers and sub-woofers as listed at PartsExpress?

thanks,
mark