re: What’s too high mileage for buying used diesel truck?

In my opinion, If you aren't exceeding the load capacity of a half ton truck there's no reason to purchase a diesel. Fuel will cost you an additional +3K each year + service and maintenance. DEF fluid sucks as well.

If you want the extra tow capacity beyond a half ton, but not diesel, you might want to consider the 7.3L V8. There's a few decent deals on used 2021 gas F250's for around $60K.