You can look at the John Deere 850 parts manual (online) to get an idea of the parts needed to mount a loader to it (and maybe the Model 80 loader too). However, the Model 80 loaders used on the 850 series are no longer available through John Deere. That leaves you with looking for a used one, which is like looking for a needle in the haystack, or looking at buying an aftermarket model. Or, did I misunderstand you and you already have the loader????
If you are looking to buy one there are FEL kits available, I think Bush Hog and Westendorf make complete kits. You can check their websites for info.
One thing to consider is, does your 850 have power steering? If not, the extra weight will make it a little harder to steer when you have a bucket fully loaded with material, especially if you are operating on hardpacked surfaces. If you have power steering, then adding a loader should not be a problem, you will just need to add the selective control valve for the hydraulics. Good luck.
