Variable Costs Question

This is my first post to this forum,,but past months of research show this site to be most likely,, the finest available to new folks in the business, such as myself and my family. I want to say thank you, to all of you, in advance, for the support I know will be forthcoming in our future.
The question I have today relates to variable monthly or annual costs tied to our startup (a few months in the future). The fixed production costs such as insurance/permits/tags etc are fairly easy to nail down. What I am most concerned about at this point are the unknowns. Now, I realize no one can give me an iron-clad answer,,,and that is not what I am looking for,,but rather,, educated and experienced inputs. Our operation will be hauling ag products under contract, mostly cotton, and only in the western half of Texas. I will purchase outright our first truck and trailer, most likely a F/L with Detroit/10-speed, with around 550K miles on it. If my research proves correct, that type truck should hopefully be the most economical, efficient, and cross-my-fingers, reliable. I do intend to pay for a thorough pre-purchase inspection. If we start out with a solid and reliable used rig,,,and pay close attention to dedicated maintenance,,,,roughly how many more miles can I expect before needing an engine or tranny overhaul? Of course a crankshaft could break tomorrow,,,but I mean on the average,,,what am I looking at for life expectancy? Additionally, if this rig were to run approximately 8000 miles per month, what should I budget for an average monthly maintenance fund,,,and then an average additional annual input to cover my major overhauls or repair expenses when they are needed? What is a good ball-park figure, per mile, for estimating tire replacement costs? Thanx again for all inputs friends,,,,I'd hate to think I had to shoot-from-the-hip and go this solely on my own. Harley48

 

Thanx Eskimo,,,and you're right,,,the info is in here,,,when I get it all layed-out and on paper,,,,then I can ask more specific questions. stay safe, Harley48