That slope in a tight area like that stinks. If you did rock it would roll down over time anyway. Some may say do it and get the money and go back charge them more to redo it.
I think you are right to cut it flat up within a foot of the A/C Condensor and the slab by the door. Install a stone wall then rock behind the wall up the building, seed or sod the rest. I do not know what your rock prices are there but here its nuts.
You mentioned that it was for a local charity, they can purchase items tax free and get donations as well for this project. That may take a little money out of your pocket but in the long term it may bring you more business.