I say skip the DIY route if you ever want to sell these. The peavey QW218 can work as a single speaker (About $650 used maybe??) + maybe a PLX3402 ($650 or so) bridged mono into the one cabinet. So for about $1300 you can have a pretty good solution that will out perform quite a few pairs of speakers. Just keep in mind that bridged 3402 will draw quite a bit of power and putting it on a circuit with other stuff could be breaker popping city.
Granted this is a starting point and you'll probably want to get a beefy amp (like an i-tech 6,000) to be able to do the QW218 on one channel, then add another QW-218 (And then more).
I have a pair of pretty high end speakers (LAB Subs) that are more or less worthless. I'm glad i made them and for what i use them for they are great, but still not worth (resale) nearly the amount of time, money and effort put into them
-Josh Billings