I'm sure you acrilic would work. I myself use Applebarrel and like paint. You don't have to primer the plaques, as a lot of people don't, but I personally think your pain job will look better. (I also refinish furniture for an antique store) so Primer is a must! lol
A lot of people do use stencils. I try to freehand as much as possible (so it is more unique) but if your more comfortable, by all means use a stencil!
I always put a top coat of clear finish on. I'd hate for the paint to be rubbed or scratched after all the work put into it. Plus I think it looks more professional.
I started off painting the dollar plaques from the dollar store, until I was happy with the results, them I moved up to the more sturdy for lack of a better word, plaques from Micheals.
Also, some plaques look better if you sand them first with a fine grit sandpaper, like 150 or above. The higher the number, the more fine the grit!