For a flat finish, mix Future acrylic floor finish with Tamiya X-21 Flat Base at a ratio of 4 parts Future to 1 part flat base. Make sure you that you mix it well, and it will give a nice flat finish. Another common flat is Polyscale's flat clear which a number of people here swear by. My local hobby shop doesn't carry it, so I use Future and Flat base.
In the $100 range, I would say anything but Aztek will work well. As an airbrush novice, I would tend to recommend a Paasche H single action. You can pick up the Paasche H Set, which includes small, medium, and large tips as well as other accesories from DixieArt for $50. Altgough single action AB's get a lot of bad press, the Paasche H is easy to use, easy to clean, rugged, and can make fine enough lines for free hand camo on 1/35 armor and 1/48 aircraft while at the same time giving broad coverage capability.
Andy