Complex sentence: Nanjing's Rover 75-based cars would be mostly sedans (saloons) and be called the MG 7 but they would also build some station wagons (estate cars) called the MG 7T.
Split into simpler sentences: Nanjing's Rover 75-based cars will be mostly sedans (saloons) that will be called the MG 7. Nanjing will also build some station wagons (estate cars) called the MG 7T.