Well, that's an easy one.
Don't buy one!
I don't really care what the specks are or what you are trying to do, I've used a fisheye twice in my 30+ year career. First while I was doing work for the military (they have a lot more gear than they know how to use and it was their lens) and the second time it was for a commercial client, the idea was good and I rented the lens which they were charged for.
That second time, I thought about buying the lens for about 25 seconds. Considering it had been about 7 or 8 years since my first use of one, I decided it was not a good buy. Meaning it wasn't going to pay for itself.
Fisheyes are in right now. To the point that people are buying things that are not even fisheyes because someone tells them it is... Fisheye photo is only useful in a very few situations. The rest of the time it is nothing more than a special effect. An expensive one.
And considering how many fisheye lenses there are for sale used out there, I would think the people who buy them often find them very boring fairly quickly.