The dress code is YDY-- You do you :) If you need more specific guidelines, something in the realm of semi-formal should be great!
There is plenty of parking at the ceremony location, Del Ray Baptist. Street parking near the reception venue is hard to come by, but not impossible. There are parking garages nearby. It is also right next to the Metro Center metro stop (accessible by red, silver, orange, or blue lines). Uber/Lyft should be easy to come by at all times.
Reagan National (DCA) will be most convenient (whether you plan to stay in DC or Virginia). It is 10 minutes away from the ceremony location. Dulles is 45 minutes outside of town and Baltimore is an hour or more away, depending on traffic.
Weather permitting, cocktail hour will be held on the roof. It might be windy and will almost certainly be hot. But hopefully it will also be lovely :)