You can use an external amp no problem, heck, it might even make the enclosure easier to build without having to worry about cutting a hole for the plate amp. I would recommend using a Behringer EP4000 as they are very powerful and somewhat cheap.
(note that I am looking to sell my EP4000, so if you or anyone else is interested, please PM me) If you want to know more about all of the enclosure stuff, I recommend downloading winISD from HTguide and do some modeling of the different enclosures so that you can see what would work best for this. You can run dozens of different possibilities in that software program.
I would recommend a different sub though as I have never been all that impressed with the JL w0's. I used to run duel JL 10w7's in a sealed box and it worked great. I wish that those JL 13w7's were not so darn expensive as I bet they would kick but, but for the money that you would spend on the 13w7's you could get some TC-Sounds LMS Ultra's and they would most likely leave the 13w7's in the dust.
As far as the slot port goes, look in the master subwoofer build thread for other peoples builds that have used slot ports. They are actually fairly easy to make, but it is hard to explain. Basically you build a tunnel that goes from the bottom of the box, all the way along the bottom then curving up at the rear of the enclosure following the rear wall up until the desired length of the slot port ends. Hope this helps, you might be able to better understand if you can find an existing build with pictures of the slot port design and build process. Good luck! I am looking forward to seeing how these JL w0's turn out! I hope that I am wrong and they really do end up sounding great!