Generations with the 6-speed automatic transmission are generally reliable when compared to the slow-moving disaster that is the GM-FORD 10-speed automatic transmission. Ford received a license to build the 6-speed from ZF for 8 years (2011-2022). If you get a later F-250 in that generation, specifically asking what transmission is in that truck. Ford did start producing some F-250 trucks with the 10-speed in around 2019. 

Well that's kind of too open of a question. Of course, if you buy a new one they cost a small fortune. I've seen people spend almost a hundred grand on one. You're going to get a lot better deal on a used one, but of course depends on what you want to use it for. Let's see if I went with 200,000 mi on it. Well if you're going to put 50,000 mi a year on it, you're going to have some big expenses in the future fixing it. But if you're only going to use light work with it and low mileage, that could be okay. Now I would say I wouldn't buy one with more than 100,000 mi on it if you want serious work out of it