Zenitar-M 2,8 16mm fisheye

Not for you if you want to use it at f2.8 and f f4 and care for (decent) sharpness. Optimum aperture is F11. A Russian photographer once told me Russian lenses are usually not sharp at big apertures, so...

But if you are planning on using only f8 and smaller: it is the cheapest fullframe fisheye on the (used) market and you do can get decent 180° photo's with it. I have experimented with the Zenitar twice on an assignment, I liked the fisheye look and got myself a very sharp Sigma 15/2.8 EX DG Diagonal Fisheye.

Its all manual focus, but it has a nice hyperfocal scale. So no need for focusing through the viewfinder if your subject is more than 1m/ 3ft away.