Need advice on body & paint $$$$$

10 grand is not out of line for the quality of work you say he can do.
A friend of mine who is a professional body man and only works on older Mopars (for years) in his restoration shop just charged 2 grand to install a full AMD floor pan in a '64 Savoy 2 door post. This was cut out the old floor, prepare everything PROPERLY, weld in the new pan, prime, seam seal, and paint. All of the materials were extra including the floor pan.
This was for labour only. Total cost was a bit under 3 grand for the owner.
You get what you pay for.