I'm traveling from Auckland to Frankfrurt with CA, stop in Beijing, need take luggage? Time to city?

1. It's an international to international transfer in T3. You don't need to take the luggage and reclaim them. BTW, the procedures are Transfer Procedures - Stamped on Passport/Boarding Pass - Security Check - Custom Inspection - Waiting & Boarding.
2. Actually, you can make full use of these 8 hours to visit Tiananmen Square and Qianmen Avenue. Then have a quick look about Forbidden City's exterior walls.
3. About the transportation, take Airport Express to Dongzhimen Station first. It takes around 30 mins. Then take a taxi to the destinations. It will spend around 10 mins. Notably, remember to go back to the airport 2 hours in advance.