The 4M50 engine has proven itself to be reliable, but having said that, there is no real way to know how any engine has been treated by previous owners.
You can obviously ask questions, but that does not really guarantee anything.
Here in Australia these tucks were often used in the rural fire service, so as much as most that came up for sale had relatively low kiliometres on the clock, that did not give an accurate picture of the engine use, as when in service they would do a lot of idling. That was standard operating procedure for a fire truck when at a fire.
A lot can be achieved by simply looking at the overall condition of the engine bay and chassis areas, as well as how the inside of the cab looks. If there are a lot of scratches and marks in the cab it may indicate it was simply seen as a work truck and not a lot of care was given to its appearance.
Check for any signs of leaks around the fluid pipe works and the condition of all of the hoses. Many of these trucks were used for snow clearing in the US, so if this was the case, the front end of the chassis should be inspected very closely for any potential damage or cracks. Speaking of cracks... the step area of the chassis is prone to cracking, so that area needs to be closely examined. A cracked chassis is not something you want to deal with.
Synchos in the gearbox are another thing to check. Changing back through the gears, especially from 4th to 3rd, will quickly tell you if you have any issues in that area. Be mindful, there is no synchro in 1st.
There is a potential issue in the transfer case, in that the design lends itself to lubrication issues, so check that there are no funny noises when driving.
Check that the front hubs lock and unlock easily and that the 4wd system works as it should. The LSD in the rear is very tight and needs specific oil to function correctly. When in 4wd, go at full lock in each direction to ensure that there are not stupid noises coming from anywhere. One note: don't do that on bitumen.
Having a mechanic with you may help, but unlike a petrol engine, you cannot simply remove a plug to check the engine compression on these diesels.
As with buying any used vehicle, it often comes down to common sense and your gut feeling of how you perceive your inspection.
Just remember, the truck is nearly 20 years old, so it will obviously have some issues, you just don't want any of them to be major ones.
