According to your numbers above, if you put a reman engine and trans into the truck, you will have $4k left for all other maintenance and breakdowns. Oil change every 10-12k, oil and fuel filters, set of tires. How are the brakes???
New trucks do not guarantee reliability or availability either. We have had several of our new trucks out of the lineup with fuel pump issues. How much does it cost to rent a truck if yours is not available for delivery???
I think the last thing I would ask is: why is the turnover so high among route owners?