The Diesel.
Pros.
It will outlast the chasis if you take care of it one million miles is the standard for diesels. Diesels also handle the heat form idling much better than gasoline engines so when your in traffic youre less likely to overheat. A hot diesel will actualy cool its self when idling. Much more durrable, and more efficient than gas depending on the year especialy pre-emmisions. 100,000 its not even broken in again as long as it has been taken care of also helps to keep your van warm when parked more metal takes longer to cool off.
Parts with some exceptions for powers stroke engines can be bought at most parts stores and online.
Upgrades are also available. Just make sure you have the cooling system and drive line to handle it.
Now the ugly.
More expensive to maintain and buy parts for So take it to a good diesel / work truck mechanic I dont care if you took auto shop in school have a diesel guy look at it he will know the issues that paticular motor is known for an un-bias opinion is typically better than ours.
There are several different models or series of any motor even gas depending on the year of vehicle take the vin number to a dealership and ask the service manager if that series engine, trans, and drive axle is known for problems make sure you tell him what you plan to do with the van.
You will need diesel an additive like howes lubricator not a big deal cost wise if you have a smaller vehicle it will last several tank fills. especialy an anti jell durring the winter I run howes year round because ultra low sulfer diesel is garbage and I value my fuel pump and injectors. With Ford Vans find out if they need to raise the body off the chasis as with F series trucks for top end work due to clearence issues with valve covers, head gaskets, injectors, and turbo . No, im not pulling your chain they actualy lift the cab and box with pickups to work on the motor.
By the way, even this in not a deal breaker a good tech can have it in the air in abbout an hour quicker on a good day. Other than that I would go with the Diesel as long as it all checks out even if they have to do a body lift.