hmm, I like the idea of finding miniature coins - nothing wrong with a mixed media approach. It will entail some work to carve off those that are already on the base, unless you just place new ones on top.
GMorrison is right in adding that only the top coins need to be fully detailed. This is something the old masters did to great effect with their canvas paintings.
Decals is another great option, again just the upper most coins need to be thoroughly detailed. I would forgo using a straight metallic paint, and rely on maybe 4 to 5 shades to give the illusion of gold colour. So for instance paint overall a mid-light brown, add a dark wash, and use decals (clear type) with two darker tones for detail. Then go back in with a fine brush to add highlights.