Most paint will need to be thinned prior to use, but occaisionally I'll get a bottle that airbrushes fine without thinning. As for the propper thinning ratio, I find that because the paint itself has so much variation, the idea of a thinning ratio is essentially meaningless. The old rule of thumbe is to thin it so that it has the consistency of 2% milk. At first it is a pain to get to the proper consistency, and you'll likely find that many of your early problems will be due to improperly thinned paint. As you start using your airbrush, you'll also likely get the hang of correctly thinning the paint fairly quickly. THe main thing is to test on a piece of plastic prior to painting your model and adjust as needed.
I've never used a strainer and have not had any problems. On the other hand, if you did have a clump of paint, your strainer would catch it rather than clogging the airbrush.
If your local hobby shop doesn't carry flat such as the often recommende Polyscale Flat, you can make your own by mixing Future acrylic floor finish with Tamiya X-21 Flat Base at a ratio of 4 parts Future to 1 part flat base. Its quite economical and make an excellent flat. With any clear flat finish, be carefull to paint in light coats. Overly heavy coats can frost up.
Andy