I'd use .080 mill finish. I'd form a pan face, then paint to customers specifications. For installation, I'd attach aluminum angles to the wall & mount to those, so no fastners are visible on the face.
http://adkinssigns.com/custom.aspx
Here are some wall mounted .040 aluminum signs I made. I bend a 2" lip all the way around the glue foam to the back to make it rigid. Then install them on alumunum track
so there are no screws in the face of the sign.