Hi Alan, the 2.5 engine was found in Sherpa 300s and 400s is either a Landrover or Peugeot unit, and won’t mate to the O/D gearbox without a conversion plate.
If your Sherpa is a 200 then go for the 2.0 perkins deisel, if you get one from a Montego it comes with a turbo and will push out around 95 BHP. I think the engine and gear box with mate, however I’m not 100% sure. I’m in the same position as you and I’m looking to replace the 2.0 petrol in my Sherpa with the Perkins, when I find out if it can be fitted I’ll let you know.
Regards
John