Hrmmm. I'll start with the sub since that's what everybody is talking about. If you want clean that you need to keep distortion down as well as transition properly with your mid. That means you want the setup to be mechanically clean. Two ways to do that on a budget:

1) Big ass speaker Less cone movement(excursion) = less stress. Less stress = cleaner sound.
2) Multiple cheap speakers to lower cone movement. Each additional speaker adds 3db and reduces the excursion/stress of each speaker.

One thing to remember though is that excursion is physically needed based on the frequency being reproduced. So it all boils down to one thing...displacement is KING. Meaning, you want as much cone area as possible.

I think an 18" sealed is a great idea. Or go find a great deal on some cheap 12" speakers.

In the end, all you need is some decent highs installed relatively "right" and one big ass sub. Get a nice 4 channel amp to run it all, two channels up front and two channels bridged and you're good to go!

Probably only need ~400 watts for the 18" in your setup so long as the box is done right. you can always build a ported based on WinISD or a similar program and then seal it up as needed so you can get an A/B comparison of what sounds best for you.