It doesn't sound like you've got much experience with plaster. The plan of applying a thin layer to plastic models isn't going to work too well. But there are materials which are more compatible that will give you a similar effect, at the cost of some detail. Check out this product from Smooth-on, for example: http://www.smooth-on.com/Epoxy-Coati...429/index.html
If it was me, I'd try this with a model I'd had printed for me by a service bureau before investing in a machine of my own, just to see if the results I got were really what I wanted.
Andrew Werby
www.computersculpture.com