M2/M3 interwork at both ends of the route, on a 2 hour round trip so requires 8 buses.
As for the MPDs, I'd likely say the 53-reg MPDs will end up replacing some of the remaining X-reg examples. As for the 71 and 34/34A, MPDs won't be used on those routes. The 34/34A will likely be going B10BLE later in the year alongside the Waggonway (and only requires 4 not 6), and the 71 will use whatever Chester-le-Street have spare. You never know what they might use the spare MPDs for.
RE: Go North East