Easiest, cheapest way to do outdoor sound (or indoor for that matter) is to pick up a cheap HTIB setup. Go with the biggest screen you can find room for. If I were shooting for something as tiny as 72", I'd probably just buy a big plasma/lcd in a few months when they are cheaper. When I started in front projection a couple years ago, a 40 inch plasma was close to three grand. I saw an ad for one on TV awhile ago at $799. You can get rear projection DLP at 60 plus inches for a little over a grand if you shop, not that I'm recommend same, but at 72" there is little motivation to go front projection IMHO.
This ain't no upwardly mobile freeway, this is the road to hell.