The engine will have to have emissions certificiaton. Thus is must be an engine which is already certified--the process is too costly and lenghty to try and get this for a new engine. The emissions certification is what has kept the Yanmar 27 and 35 hp diesel outboards out of the US.
The Mercedes 3.0 Liter engine used in the Dodge Sprinter might be the larger of the two engines. There are a series of Mercedes engines which are from 1.8 to 3.5 liters--including a 2.5 liter--which may well be the smaller of the two outboard conversions.
I would think that gearing and prop size might be a problem with the diesels, since they are going to turn slower than the gas engines, that the lower unit was designed for. Modified Merc outboard units are used up to 500 hp in racing boats--so I suspect that they can be made robust enough.
I didn't see any prices--but I suspect that they aren't going to be cheap!