GMC 1997 Utilimaster P3500

We run an urban farm and are looking to purchase a diesel step van to be used as a “mobile market” that roams the streets selling fresh produce in urban neighborhoods. We have found a few GMC 1997 Utilimasters for sale that were recently decommissioned from the postal service, bough at auction, and now being re-sold. A couple of these vans have only 30K miles, which makes one wonder why they were decommissioned. An employee at one state inspection place said over the phone these vehicles have “bad engines” and advised us against purchasing one. The engine is a 6.5 Non-Turbo Diesel with a 4-spd Automatic Transmission. The body of the vehicle is exactly what we are looking for, but we want to get something that will last. Any thoughts on these engines?