Cummins LSM.... Determining It's Condition.... ??

For 20k miles yearly, keep.

 

If maintained properly, maybe. Not a truck I would buy, but not one I'd replace until needed since you have history with it.

Owners may need to spend some money before the end of the year too

 

I'am not a Cummins fan at all but the L10 is pretty durable
Got 1 in the shop now that in a Farmyard Ferrari thst been beat to death and back
As @Big Road Skateboard said at 20k miles a year ..keep it

 

I would expect 6 or 700,000 miles plus.
There is a very similar truck here that was inframed at about 375,000 miles. It had been ran hot repeatedly, they have the oil changed possibly once a year. It is a straight truck with 4 axles total and a forklift hangs on the back.
A long block exchange was priced at about $24,000 about 3 years ago. It was inframed for much less about 2 years ago. It is still running and dependable everyday.
A friend in the neighborhood has a new truck that was in the dealer shop 200 miles away for 2 or 3 weeks. They were waiting on a sensor.
If you are driving it and like it take care of it. Ask them for a raise to keep driving this truck because you are saving them money.