The problem I have with any diesel engine in a "lawn mower", including zero turn mowers, is that the motor will our live the deck and drive system three or four times over. We have sold many diesel GrassHoppers to the state and couties and some have over 6000 hours on them. However, once they get up to around 1500-2000 its time for a new deck. After the deck goes out the clutch and drive units won't be far behind. It was looking at one of the units the local county has, 6013 hours, has had four decks, three drive units, and three clutches replaced mainly because the motor is still holding up.
As a homeowner I couldn't justify spending a few extra thousand dollars for a diesel motor because chances are you will never put the amount of hours on it to justify the cost. Diesels are great for people that use them every day to make a living, but other than that they are overkill. Thats just my take on the matter.