I don't think you'll save much money with DIY, it's more about the fun of building it and the way you can customize it. If you just want the best bang for the buck look into ElectroVoice ZLX. Looking at that speakerhardware.com site you linked it's still going to run nearly $350 for a 15" 500W kit and the 15" EV ZLX 15p is $400 and it is 1000W.
https://www.amazon.com/Electro-Voice-ZLX-15P-2-Way-Powered-Loudspeaker/dp/B00CE0VN3M/
That thread is informative, but I think 10 years old. Better products now exist, though the general gist of the thread is still correct. If I was getting something, I'd go for this. It's relatively light and very powerful and clean sounding. Electro-Voice ZLX15P https://smile.amazon.com/dp/B00CE0VN3M?tag=amz-mkt-chr-us-20&ascsubtag=1ba00-01000-a0049-win70-other-smile-us000-pcomp-feature-pcomp-wm-2-wl-ask0&ref=bit_pcomp_smile
If you're really short on money, really anything would work, but to make it sound good it's going to need powerful bass and plenty of amplifier power. That simply does not come cheap.
JBL and ElectroVoice are my personal favorite brands.
If you don't find anything one other option would be to buy a single decent speaker right now and add another speaker and sub as you can afford it. Yeah you'll have to run everything mono but if this is unpaid house parties then no big deal. If you go this route I suggest the ElectroVoice ELX15p for $499: http://www.amazon.com/Electro-Voice-ZLX15P-Two-Way-Powered-Loudspeaker/dp/B00CE0VN3M/ref=sr_1_3?s=musical-instruments&ie=UTF8&qid=1421962417&sr=1-3