If you want good sounding them maybe an Image dynamics idq 12 or id 12.
http://www.sounddomain.com/shoplist~...s~t~Subwoofers
Just to get an idea some may be on backorder from there but they are the only authorized place online if you don't have a local dealer. Id say maybe 1 12 for now and then if its not enough you can get another later on when you have more money. Just make sure the box is built to specs. Its very important for a sub to have the proper box built for it. If you want more output maybe a ported box before you buy another 12. Bad boxes can make good subs sound bad.
As far as an amp this one shop said their installer sweared by these amps. By no means top of the line but very good amp for the money.
http://www.sounddomain.com/sku/COU401SE
These are just some suggestions. I think it is way better equipment than the audiobahns but will probably not be as loud with jsut one 12. If you have the budget you could just go with two 12's or 10's. That amp should have enough juice for a pair. It might be a bit over your budget though with install.