I'd get my local CNC guy to cut them out of 3/8" PVC (hdu would probably be too expensive thought the router could give some neat details), spray the flower yellow and then apply a digital print of the center portion of the flower. The stem could be plastic tubing and the leaves probably gotten from faux flowers at an arts/crafts store.
If you are stuck on metal then 1/8" aluminum with aluminum tube stems (welded to flower) and again plastic leaves from faux flowers. I'd stay away from metal myself because of sharp edges, etc. Also, I'd want to know their idea of long term. They might be thinking of anywhere from a couple of months to a couple of decades.