That's going to be rough on one or both of you. The commute will be a killer.
I'd suggest something close to D.C., either in Virginia or Maryland. The problem is there are only three ways to get from Virginia to Maryland: The American Legion Bridge, the Woodrow Wilson Bridge, or through D.C. Working in Fort Belvoir, that rules out The American Legion Bridge. And you probably wouldn't want to go through D.C. So that leaves the Woodrow Wilson Bridge, unless there's some major tie-up on it...in which case you'd go through D.C. as a back-up.
Alexandria is a good possibility. Areas of it are right on the Beltway or south on 95. It's relatively close to Fort Belvoir. There's plenty of mass transit. There are all types of communities, so you probably could find housing you'd like. The drawback is that your husband would have a relatively long commute and would have to cross the Woodrow Wilson Bridge.
Prince George's County, right across the bridge in Maryland, is the other logical possibility. It would even out commuting times somewhat. There's a lot of affordable housing in PG County. Areas are very close to the Beltway (495/95). The drawback is that (as far as I know) there isn't any viable mass transit from PG County to Fort Belvoir. Basically, DC is a hub, with the exception of the Metro--the subway system--not much goes through from Maryland to DC. There are some nice commuter rail lines--VRE in Virginia and MARC in Maryland, but they don't cross over. They both end in D.C.
If you're still looking, you really might want to consider employment on the Maryland side. If not, then Alexandria and PG County are your two best options.
Hope that helps.