OP - Another longtime hostmom here. I think there is one aspect you should consider, that maybe you haven't yet...
If you are a host family in NW DC, your au pair will have easy public transportation access to everything -- plus, her cluster of au pair friends will all mostly be in the same boat.
I am familiar with the areas of Bethesda you are describing, and to me, it is a decidedly SUBURBAN (rather than urban) environment. While you may live close to a bus line (where she can take a bus to the mall, or the metro), many of her au pair friends are going to be in other suburban parts of Bethesda that will not be easy to get to. I think it is essential to a good au pair experience (by both the HF and the AP) that the AP 1) have AP friends; and 2) have sufficient ability to get out of the house and meet those friends.
Your environment is not super-conducive to #2. That's OK. You were up front about it, and the reality of your family is that you share one car. But just keep in mind -- if your AP is from many parts of the world, or an urban environment -- she may have accepted thinking "sure its OK to use public transportation, I use it all the time here), and then she gets here and realizes that taking a bus to the metro station, so that she can pick up another bus, on the weekend, when they only run once an hour, so that she can see her AP friend for 3 hours -- well, that's a strong disincentive.
I am in another suburban area (slightly smaller than Bethesda) and we bought a 3rd car (a somewhat junky older car) specifically for AP use. I realize not every family can afford that, or wants to do that. But I did it because I realized that if I needed to get across MY town, to visit a friend, I would NEVER go out if it meant taking 2 buses to get there. I'd encourage you to think about that. When YOU go to visit your friends in Bethesda, what do you do? Maybe you all like coffee so its no problem to just all take the bus to Starbucks. But if visiting your best friend meant taking 2 buses and 90 minutes to get 4 miles, would you do it?
If you wouldn't, then I don't think you should expect that she will either. It may be a non-issue. Maybe all her AP friends will have cars, and they'll pick her up. But -- a housebound AP will be an unhappy AP. An unhappy AP makes for an unhappy HF.