Why is DEF bad and why should you look into buying an older truck without it?

As the title says I’m curious to know why that is? Not looking into becoming an O/O yet but I’ve always been curious on how y’all run things. From an owner perspective what makes the perfect truck for you? I’m just a company driver and want to get more input on this from O/O since I don’t know much about the mechanical side of this trade but I want to learn more.

Another series of questions is what should you look out for when buying a used truck? What’s an engine overhaul? What are some basic mechanical things to know how to fix and how to fix them? How much is the perfect amount of money to finally buy a used truck/new truck? Do you value more in cab space above everything else? Should you invest in just the truck and not buy a trailer or might as well get both? What tools should you always have on you? Automatic or manual? Why do people get upset when someone drives a automatic but knows how to drive and back up perfectly apparently you are not a real trucker this one is separate but I never understood it lol