RE audio, Sundown, DC Audio , Fi Audio , Digital Designs....
You Could really go crazy and get real loud with such a big area to work with...few 15's would be great all the above brands are very good ones amp and sub wise
im working on a project: 3 re audio sxx15d4 (would do more but the area im working with is 1/2 yours)...you can always start with 2 15's and add more in time but bigger the box louder u will get. the subs im getting require 4cu each so my box is gonna be 45in x 40in x 17in roughly 12cu after all the displacement just enough for 3
if u really got the cash and are willing to so look into the 18's since u got the space to build a massive box......
hope this helps man. maybe someone on here lives close and might be willing to meet up and help u build a box cheaper than a shop