I agree with what others have said above - mirrorless is a good option - particularly the higher end Panasonic M4/3 camera models. Good thing about a lot of the Panny G series cameras is that they allow full manual exposure control during video mode - which is one of the most important features to have with regards to shooting video. Another nice thing about mirrorless is that with adapters, you have access to a massive range of lenses including lenses designed for the 35mm still film format, medium format, video lenses, cine film lenses and even enlarger lenses etc. And of course native AF lenses too. I regularly use Canon FD lenses from the 1970s and 1980s with my Panasonic G2 and G6. Ive also used some of these same lenses with my 35mm SLRs. Got some nice HD footage of my local city at night with a Canon FD 50mm f1.8 wide open - shot with the G6. By the way, I wouldn't recommend the G2 for serious video work as that only records 1280 x 720p and doesn't have full manual exposure control during video recording. Just recently got a G7 - a much better camera for video.
Another cheap option for fisheye lenses are Russian made optics but quality control can be a bit variable. These often come equipped with an M42 screw lens mount. I had a Peleng 8mm fisheye but I wasn't all that happy with the sharpness (especially wide open) so I sold it. An ND filter will also come in handy for reducing your shutter speed (50th for 25fps and 60th for 30fps) though these would be tricky to use with a fisheye lens. There might be some drop in filters available that sit behind the lens.