Whether you buy a mountain bike, a traditional touring bike, a hybrid, or a recumbent, my advice to you is to find a bike that fits you. You are going to spend a whole lot of time riding this machine and there is no sense in being uncomfortable while you are doing it.
Ride before you buy. Any good bike shop including REI will let you do this.
I commute most days by bike in DC and I use either a Tour Easy recumbent or an old Specialized Sequoia Touring bike. My quess is that recumbents will generally be out of your price range. I love mine and wouldn't thhink of touring without it unless I was going off road most of the way.
You should also consider a few essentials such as braze ons for attaching essentials (racks, fenders for and aft,and water bottle cages). A wide range of gearing is a must. Then you need to think about all the stuff you will be attaching and carrying. A lock, pumps, basic tools, panniers, etc.
But more than anything, fit is important even if it means spending a few dollars more. Many years ago I went shopping to replace my trusty old Raleigh Grand Prix 10 speed. As I test rode the lower end chromoly road bikes, my wife took the sales clerk aside and said "Sell him a better bike." I ended up spending about 30 percent more than I planned for an alluminum Trek 1200. I rode it for about 10 years and never regretted the extra expense. (I sold it to a freind who is still using it 5 years later.) So if you really plan to ride a lot (and it sounds like you do) a nicer bike will be worth it in the long run.