First of all: thanks for all the input so far! The body is at 400k clicks and lacks some paint but otherwise looks good. Price is at about a third of the regular going price for a used 1DXmk1 and it's getting a checkup by the store. I'll even get some warranty on everything but the shutter. So for argument's sake, let's assume it'll function as it should. My main interest is in whether or not I'll want to use it. I'll keep the 80D, by the way!
My main concern is with IQ and reach. I'll be losing reach twice, both in the crop factor and in the loss of cropping room by going from 24MP to 18MP. On the other hand, my 24-70 will become more useful/versatile.
In terms of IQ, I have very little to go by. DXOMark appears to prefer the 80D sensor for all but high-ISO, but I'd need to get out there and shoot it for a few days to know whether or not I got what I need.
Main question for IQ is (in my mind, but I'm open to suggestions!): will the fact that the 1DX has a FF-sensor outweigh the fact that it's older and lacks resolution?
My stuff goes all over the place. I love hanging out at the zoo, shooting animal portraits. But I also shoot parties. I have a little studio setup at home and have shot the occasional wedding. Not big on landscape, but that's mainly because I suck at it! I've attached a few pics I shot with the 80D to give an impression.
Would the low-light AF speed and accuracy of the 1DX be better than my 80D? I do get lots of missed shots during parties because the AF simply doesn't manage. I now use the focust assist beam on my flash, but I much prefer using Servo AF with dancers, as they tend to move around quite a bit