transit in Beijing Airport

Hi
Im going on a trip to US from Taiwan and my transit is at BJ airport. The period is only 2hrs. Im taking Air China from Taipei, Taiwan to Beijing and United Airlines from BJ to Washington Dulles. I booked the tickets on united.com. They both are Star Alliances members.

My quesitons are:

1. Would I get boarding passes for both flight in Taipei or do i need to get the pass for my second flight at BJ ?

2. Do I need to collect my luggages at BJ airport or can I arrange in Taipei for my luggages to be collected at the final destination?

3. Do I need to go thru immigration at BJ or do I go straight to the gate for my next flight?

4. Is the 2-hour transit enough?