I would recommend that you look for a good used boat. I purchased a used Shoalwater several years ago for $3200. It is 15ft with a 40 Yamaha. While this is smaller than what you are looking for, it gives you an idea of what kind of a deal you can find. Mine was a winter Texan boat that had barely been used. One good thing about a used boat is that it usually already has a few scuffs and scratches. So you can enjoy fishing instead of hating the latest bump into the dock.
The second thing to consider is what you want in a boat. If you want a stable platform for fishing, I would look hard at the flat bottom, wide, partial tunnel like a Shoalwater or Carolina Skiff. The down side is that with the flat nose they are rough riding when under way. To get a better ride look for a semi-vee with a flat or rounded bottom like the Blue Wave. The trade off for the better ride is less stability when anchored. The curved bottom will roll with the waves. If you like fishing the flats there are some boats that have very little draft, however they probably will not fit into the reasonably priced category. You will have to consider the things that are important to you, this will eliminate some of the boats that are available and then decide what you must have and what you would like to have.
Hope that helped.