Hi,
A minimum of 2801 will do. With CCME you can connect directly the IP phones or the existing PBX for the migration, and a variety of data interfaces, like the dual-T1 that can be used for voice and data simultaneously. You will need PVDM2 DSP card to support all the simultanous conversation that you plan to have on the PRI.
Hope this helps, please rate post if it does!