New O/O Here - Seeking Accurate Maint. Figures

I'm starting a trucking company. I have 1.5 years of driving experience for a company. But most of that experience is in a daycab so going over the road there will be a learning curve. My only OTR experience is 2 months of training.

I'm not getting my own authority or any of that. Just getting an LLC and a truck and going to drive for my friend's new company, they're going to charge 10% and give me 90% of the load revenue and send me wherever I want to go, whenever I want to go. In the beginning I plan to work very hard 6-7 days a week and stay out on the road and then start buying more trucks and hiring drivers. I've been quoted at $1100/ month for insurance

I would love to get some feedback on how much y'all think maintenance and repairs is going to cost me on average. I heard that I should factor 7 cpm for maintenance. So I'm figuring on roughly $800-$1,000 per month on maintenance and repairs. Does that sound like an accurate average figure? I won't be doing much of the maintenance/repair work myself, I'm sure that makes a big difference

Also wondering what kind of truck I should get for my first one. I think I'll be going used in the $60-90K range because I heard that as a first time truck buyer I'll be looking at a 20% down payment so that would be at least $30k for a new truck. I've been looking at some cascadias with 350k-500k miles in the $60,000-$70,000 range at a freight liner dealership. Does that sound like a good idea/fair? I also test drove a 2015 coronado glider kit with 700,000 miles. It was a blast to drive but I feel like $70k is asking too much for that many miles. What is a good first truck to buy that will get good fuel economy and be reasonably reliable but not break the bank? Cascadia? Peterbilt? T680?

Thx and keep on trucking, glad to be a part of this forum