thinning paint??

Jeremy:- I use only acrylic paints. As a matter of fact I use only Tamiya acrylics, so you can judge my answer based on that. Any acrylic paints can be thinned with water. You can use tap water, but I recommend distilled water. You can buy it for a few bucks a gallon. You can also thin any acrylic paint with 70% isopropyl alcohol (rubbing alcohol). It's available at drugstores for not much money. The alcohol will allow the acrylic paint to dry much faster than water will. If you're using Tamiya acrylics, I'd recommend their X20A thinner. That's the best out of all the thinners I've tried.
Of course, the best answer is to test. Get an empty margarine tub to practice on. That will give you a good idea of how the paint will work on your plastic model. Good luck.
Rich