Advice on a PA build

My dad wants me to build him a box for 2 18" subs to use for his PA system for his band. Its only for the kick drum, and maybe the bass guitar if he can get the signal into it, anway, the easy part is building a box, but what subs should I get? Price is nothing over $300 a sub. I was thinking maybe Fi IB3, or something, I really dont have a clue, It will be given like around 1000-1500 watts im assuming, or he could dump 3000 watts into them. So what subs would work good in a PA type thing, any subs?

maybe 18" PA speakers?

http://www.parts-express.com/wizards/searchResults.cfm?srchExt=CAT&srchCat=583

If its just for drum and maybe guitar it wouldnt really need to get sub 30hz (could be wrong) but these would definitely be more efficient than the ib3. Plus I think to get that IB3 to play right you would need a pretty big box.

If you end up building something for this post up some pics of the build, I'm curious about the results

Thanks, and If I do build a box I will. We found a nice DJ system 18" sub box for cheap we might just buy

Just a lil advice, If you're buying it used you might want to check out the speaker really good because alot of those dj systems get ran to the point of clipping and get abused by too much volume