Haven't attacked the engine yet. Since it actually gets around OK on 5 cylinders I am attacking the camper bits to get it registered as an RV in AZ. But I am on the lookout for a donor vehicle with a low mileage Cummins. I was leaning towards the 450 HP ISL but now my thinking is a 2004 EPA ISC because they didn't have any EGR setup (I think). Swap shouldn't be too hard especially if I manage to get a wrecked RV as a donor.
I can see where you're at on this -- on the one hand, there is an engine installed that (mostly) works, and that should/could be repairable - but on the other hand, if you go to a Cummins, you'll have an engine that everyone likes and respects. Parts availability for the Cummins would probably be better throughout the world than for the Mercedes, plus a salvage yard is bound to have more Cummins parts than Mercedes.
One of the deciding factors in my buying the ambulance that I did was that it had a Cummins. I didn't want a CAT engine, I didn't want a Detroit Diesel, and I had heard nothing good about Mercedes from the time frame I was looking at. I talked to several ambulance mechanics, and they all said to go with the Cummins.
I went with Cummins, and I like it.