Well for single din non-video units i personally would only go eclipse or alpine. if you went w/ a dvd unit(obviously a bit more) kenwood excelons are good and i even liked a Panasonic one i had. some people like pioneers but some comes down to personal preference on how you like it.
Dual ohm woofers have advantages in the versatility in how they can be run on different amplifiers. say if you have a dual 4, you can run it at a 2 ohm load if your using it by itself(just make sure the amp you get runs 2ohms - most do well these days), or you can run it at an 8 ohm load if you ever add another. then 2 subs runing at 8 ohms ran together will split to make a 4 ohm load on your amp. or if you get 4 subs you can get it to a 1 ohm load etc.
And for the box, ports, slot ports, sealed will all give you a little different sound. with one twelve id recommend ported to get a decent amount of output. will sound well in sealed but you my find yourself wishing you had a little more output. Again if you go with a dcon ill let someone else give the final recommendation on the box. Ive had good success building myself some slot ports, but not 100% on the best setup for the SSa's
what would your amp and head unit budget be?