I can afford to be selective now and would avoid anything with over 25k miles. But from my personal experience of buying 4 used trucks in the past 10 years id try to target them in the 50-75k mile range and preferably the lower of the two. Towing loaded trailers beats on the trucks and reveals their weaknesses very quickly. You can go with very few repairs the first 75000 miles but going 75000 to 100000 the repairs start to add up quick. Your block is fine the pistons are fine its the components that fail. Air conditioning, alternator, power steering , air pump, water pump, radiator fan. All have bearings and all have a limited life and when you get to 75000 your getting close to finding it once the belt snaps your dead in the water. The brakes are showing their age suspension and all the springs and ball joints are getting close. Exhaust is often rotted even if you cant see it yet. Then we get to the fun ones rear end drive shaft and transmission and 4wd cost a fortune when it fails. And that still leaves body rust and frame rust and all the interior issues that arise on a work truck after you put the miles on it. And I am 4 for 4 iv yet to ever buy a used truck with decent tires.
My experience is 120,000 miles and up its not even worth owning the truck any more. The resale value is near nothing and you feel like and ass every time you sink another grand or 2 into it which if your working hard can be every month or 2.