The company I work for just got our Basic 135 approved and I had a big part in the process. It was alot of work, but I learned a ton. I would check into some of the resources these guys have put out there. I think the single pilot/single plane operation would be a good place to start. That's what we were before we got approved for the basic. You don't need a GOM (General Operations Manual) , or a Letter of compliance, and you don't need a training program. I think somebody already said it, but the OpsSpecs come from your FAA POI that is working with you. I just outlines what you can and cannot do.
With our basic we can have up to five airplanes and five pilots. As far as figuring operating costs, I think that some of the things that go into it are the insurance, hangar, maint., fuel, pilot(s), and maybe a couple other things.
I'm not completely sure on that.


Good Luck and I hope some of that helps

Papa T