I think you'd be fine with anything from a good single 10" to dual 10" or a single 12". Any of those three are going to be able to give you what you are looking for if you do it right. I'd probably go with a simple sealed enclosure as it is the easiest to make sound good. When you start messing with ports it becomes much more complicated. You can't just throw a port in any box and expect it to sound good.
You might check on craigslist. You have to be careful on craigslist to not get trash but there are some good deals there especially with people trying to dump their systems to pick up a little extra cash with times as hard as they are for many people. If you don't know a whole lot about it though it might be hard to find the good (quality) deals.
Try to stay away from the "cheapie" names like Dual etc. Try to stay with something decent like Pioneer, Rockford, JL Audio, Clarion etc. Those are some of the big names that have some entry level stuff. There are lots of good companies out there but not many of them are quite as mainstream as the four I mentioned above or they are focused on the really high end market.
