7000 feet really is big, but it doesn't have to be too big... Just approach it with realistic expectations.
I have my home theater in a 2700cuft living room that has a 6 ft wide doorway which leads into a 1000cuft dinging room on one side and a 400cuft foyer on the other side. All total its just over 4000cuft.
At the moment I have 1 sealed 12" SVS SB2000 corner loaded and I am very satisfied. I am going to add a second sub in the future just to even out the response, however the bass levels from the 1 sub are more then enough for me.
If you want a theater that plays comfortably at reference levels with earthquake levels of bass, you're going to have to buy atleast 4 subwoofers. However if you are realistic about your expectations you can start with one sub, then purchase more later as your budget allows. Speakers aren't like PC parts... a lot of manufactures will keep the same product lines for years as opposed to months, so you have time.
If you do it right your home theater can last you years, so approach it the same way you would home renovations. Be patient take your time and build something that is going to be acoustically and aesthetically pleasing to you. If you try to do it all at once, it will just be something that you are going to try to upgrade again in a year and will end up costing you more.