If you're asking these questions because you're interested in buying a truck, you better know damn well what you're getting yourself into. It's almost impossible to make money as an owner/operator these days. Any good truck owner knows exactly how much it costs to run his truck per mile, down to the tenth of a cent. And remember, it all depends on the routes you will be driving and even what loads you'll be hauling. Every route has a different "rate." If you're in the hills you'll be burning more fuel. If you're hauling heavy, your driveline components, brakes and tires will wear out more quickly as well as use more fuel.
Also, if you think you'll be able to do your own maintenance, you may be able to, but you might also be giving up part or all of your warranty by doing so. And any time spent working on your truck, I think is supposed to be logged in your logbook as On Duty, Not Driving. So there goes your 36- or 72-hour reset.
The best option nowadays, if you can get the financing, is to buy a truck that sill has a good chunk of warranty left on it. If you don't have a warranty, and you blow a turbo, or have any other problems that will put you in the shop, you'll not only have to pay for the repair, but you won't have any income during that time. Makes it kind of hard to cover the truck payments if you're not rolling.
That being said, trucking is a decent way to make a living if you can do it right. I'd like to hear from people who have more specific answers to your questions, though. I drive a company truck, so I don't have to worry about any of the expenses. As long as I get good fuel mileage and don't destroy the equipment, the company is satisfied.