What is the best way to get to the Huairou District from the airport in the evening?

I have a 21 hour layover in Beijing. I plan on staying in the Huairou District, going to the great wall early in the morning and then leaving before noon to catch my flight at 15:35. I will arrive to Capital at 18:10, after going through immigration I doubt I will have time to take the Airport express to Dongzhimen Bus Station and take the last bus to Huairou District. Thus can I take a taxi to that district, if so how much would it cost?
Additionally, since my flight leaves the next afternoon. What is the best way to get to the airport and how long will it take from the great wall?

Thanks for your assistance.