On average the weather in Thurmont Maryland in November is approximately 57° during the day and 42° at night.
The reception is indoors! The Ceremony will be outside, but briefly and at 3:30 in the afternoon. Hopefully, it will be around 57-60 degrees Fahrenheit for the ceremony. If that is still super cold to you, feel free to dress in a nice warm coat or a lovely faux fur! ;-)
BWI is the most reasonable, however, Dulles is also an option as is Regan in DC. Those will be further away though! My choice is always whatever is cheapest and easiest! We do have mass transit and a lovely rail system in Maryland though, so if you do choose to arrive in DC (it's lovely that time of year) you can take the Marc train from DC to Baltimore and then I would rent a car or catch a ride with one of us to Frederick!
Although we love to watch the children run and play, this is going to be an adults only kind of day.