* AF is not absolutely necessary with fish eye's... DOF is pretty deep
* Certain fisheye's might not get you the results you expect on a crop APS frame.
* Fisheye glass that screws to the front of another lens via the filter threads are not ideal but they are inexpensive.
* I don't believe "cheap" and "fisheye lens" usually make it into the same sentence. No more than "cheap" and "super telephoto"
Your best buy are M42 mount Zenitar and Peleng. If your lucky a Pentax/Takumar.