I would start at 15000. You can adjust it from there. When I was working for a fleet we had 50 T600's with N14's when we went from 12k to 20k the add oil went up significantly. I think if the bean counters would have put it on paper we didn't save any money.
I can certainly understand where you are coming from. God knows I've had the same issue with car dealers, including some I not only refuse to work with but actively steer other people away from. The dealer I'm working with, however, has been endlessly helpful, always ready to answer the dumbest question I can come up with, and endlessly patient with me. They are actively working to not only make me happy, but to retain me as a future repeat customer. I appreciate everything they are doing, and will be posting a full review once I've completed the full process.
My banker is VERY HAPPILY smiling at me. As a personal bonus, she's also single and seems to find me charming. (Hey, I'm ugly! Charm is all I got going for me....and a slightly twisted sense of humor.)
Mine are not too bad. They have been pretty decent considering. I watch every penny a month or two ahead. Essentially the income is already spent 3 months ahead. Sheesh.
After ten years of busting tail and tightening the old belt, I now have both liquid capital and leverage-able assets. And a fantastic credit rating. I'm in a great position financially, and have the ability to completely recover if everything goes totally pear-shaped on me. One of the biggest failure points I've seen with new Owner/Ops has been the fact they go whole hog and dump everything (including things they can't afford to lose and shouldn't risk) on a dream sold to them by some lease purchase recruiter. My home is paid for. So is my car. I own all the toys I actually want. My credit card debt is minimal, and will be zero by the end of the month. My only outstanding debt is a couple of student loans from back when I bought into the college BS, and those are not only caught up but paid a couple of months in advance. I'm in my early 40's, in better health than I have any honest right to be considering my lazy lifestyle outside of the truck, and have been working towards this for over half my life, even before I seriously considered it to be a possibility. And thanks to the very knowledgeable people I have been lucky enough to know over the course of my life, I believe that I am now ready.