One wonders whether it's really worth their while repairing whatever clapped-out "heritage" diesel unit they're proposing using. For that much money, they could have bought six class 139s, which would actually have been reliable (so they could donate four to other branch-line reopenings...).
Class 139, Wareham to Norden or Corfe Castle - issues to address:
1. Need to be isolated from main line when in passenger use (not suitable to share with heavy rail)
2. Need a capacity of rather more than the 25 to 30 seats I think they have
3. With the flywheel power, do they have the range necessary or flexibility to shunt?