Gotcha. There are many ways to accomplish this. I'd personally get a single 15" driver, just to keep enclosure space down. Either way, it's going to be a large enclosure, but probably not as big as one for two 12" drivers. (guesstimating)
Yes, you can get both. There are guys with dual 15's in big ass ported enclosures with excellent sounding stereos (like sound quality competition excellent).
^^ I think you just contradicted yourself, lol
When I say 'driver' I am talking about the speaker. It's just another term for speaker.
That doesn't make what you said wrong though... as long as the person is happy with their stereo, then there is no right or wrong.
As far as ported vs. sealed;
Now... look at these graphs I modeled below.
Green line = ideal ported
Yellow line = ideal sealed
Pink line = ported (what most people build when they are looking for loud
The second graph shows group delay, I'll get into that in a moment.
So, lets compare ported vs sealed. Looking at the first graph it's pretty obvious that the ported alignment will pay lower than the sealed alignment. Those lower frequencies will be louder with the ported enclosure compared to the sealed. This is why people coin the ported enclosure as being louder. It is, but only at the lower frequencies. You can see it is just as loud throughout much of the lower frequencies.
Now, that low-end extension you get when going ported comes at a cost; group delay! This can be a real bitch to overcome for some cars, others, not so bad. The easiest way to explain it is that those lower frequencies that coordinate with the rise in both the green and pink line are going to essentially be delayed. This can sometimes be combated with a simple phase swap (reverse the positive and negative wires), but other times it takes a bit more processing to smooth out.
The other, pink line, is what a lot of pre-fabricated (Kicker, Alpine, Rockford, etc) enclosures model at. Very peaky = really loud at essentially one frequency. In my opinion, it sounds like shit, but others that just like a loud stereo and don't necessarily care about a smooth/flat response prefer a response like this.
To conclude: sealed enclosures are much more forgiving and are easier to implement. Ported enclosures give you the opportunity to shape the frequency response of your sub woofer. Of course, this gives you the opportunity for it to play lower (louder) - better sound quality (this is the basis of that argument), but group delay can cause issues, not always, but it can.
That's about all I feel like typing right now, but if you have any other questions or need further clarification, I can try to help answer some questions.