I would tend to agree with PNW Steve that the price you will have to pay for a brand new unit is going to be a lot more than what you have suggested. Your option list is not cheap.
If you get an FE you can't have pass through under the floor luggage compartments--the driveline gets in the way. All you can get are skirt mounted.
I am not familiar with which engines are available in BB's these days. I think the only one available in the FE is the Cummins ISB. There is a CNG option but I don't think you want that.
Not sure exactly why you would want air disc brakes over air drum brakes. They are an expensive option with relatively no difference in performance with much more expensive repairs down the road. You can usually get new drums, shoes, pins, and bushings for both sides of one axle for the cost of one disc.
I think that if you use the WA state school bus quote you will be able to price out a bus pretty close to what you will have to pay for a one off bus. Or at least you will be able to see what the options will cost.
Good luck and happy trails to you!
2017-18 State Quote Specifications