Are there taxi or vans available at Beijing Capital Airport?

Yes, you can still find several cabs at PEK. Don't worry. However, it won't be easy for you to hire a private car at that time. Generally speaking, one official cab can accept four adult passengers at a time. If the size of your luggage is large, you'd better get two cabs from the airport. BTW, for one cab, it costs CNY 120-130 at night.