The PE kits are OUTSTANDING. You cannot possibly go wrong there. At 400 you can pick up a Titanic 10" with 250wat RMS amp. Or go nuts at 900 with the dual 12" 950W RMS job.
I went with:
http://www.parts-express.com/dayton-audio-15-ultimax-subwoofer-and-cabinet-package--300-7097
for $268.....
Pick whatever amp you want to go with it after that.
I went with (But you only need the NU3000 to run it for $230)
http://www.parts-express.com/behrin...0-watt-four-channel-power-amplifier--248-6800
for $350. Requires a fan mod to make quite which I have not done. Why? My Subs are hooked on this amp. If it's on, my subs are on, I can't hear the fan 95% of the time so really don't care. Otherwise, I just keep it off. It also runs my surrounds on the other 2 channels. I just listen 3.0 if I'm just browsing the internet and turn on the subs/surrounds for music/movies.
That's just what I did though and to tell you that the experience finishing has been great. Far better than what I could have gotten purchasing a $500-600 sub.
I'd say definitely build if you have the time and the money and the equipment to do so. Since I know I'm building every piece of my system after building my subwoofer (and I've followed DIY speaker community for awhile so I've been learning about how great the speakers are and the SEOS speakers), I felt the equipment purchase was well justified considering how little equipment you need to assemble a speaker kit or subwoofer kit.
It's really whether you want to spend the time to do it really. It's not hard, it's extremely easy if you've ever used power tools before so I'd definitely recommend doing it.
It's really how hard you make it depending on the finish. The harder finish you choose the harder it is. If you just throw 2 coats of duratex and call it a day you have a great looking speaker/sub that was easy to build and performs amazingly well compared to the price you pay.
Personally I recommend you start with a 15 inch or 18 inch sub from the dayton ultimax line but that's probably out of your price range.