This is a dressy casual affair. No jeans. No fancy dress. Ties not required.
Yes, Dulles International Airport (IAD) is a 45 minute drive to the venue without traffic. This is also Metro accessible through the Silver Line to Rosslyn station wherein you transfer to the Blue Line to King St. - Old Town.
WMATA accepts contactless payment using your contactless credit/debit card (physical or mobile wallet). You can also visit a fare machine to purchase your SmarTrip Card. Visit https://www.wmata.com/fares/how-to-pay/index.cfm for more information.
You can use rideshare apps like Uber and Lyft. You can also use Water Taxi from The Wharf DC, Navy Yard, Georgetown or National Harbor. You can find more information about Potomac Water Taxi here - https://www.cityexperiences.com/washington-dc/city-cruises/water-taxi/ . You can also use the city bike. More infomation here: https://www.alexandriava.gov/Bikesharing . Another intercity rail service option is Amtrak. Alexandria (ALX) is served by Amtrak or VRE trains. The station is right next to the King St.-Old Town Station.