The 855 I traded for a one of the very first ever made (54th off the line). It is equipped as you mentioned with loader in front, mower underneath and box blade in back. All the other hydraulics front and back work well but struggling to figure out getting that deck raised out of the way. I've been reviewing the JD parts site but can never be sure if it is the right one needed.
Interesting. My 855 was also one of the first ever made, I think the serial number ended in 056, but will need to look that up. One of the problems you're going to run into with this tractor is that Deere made substantial changes to many components in year 2 of manufacture, ca.1987, so many of the parts on your machine are different than nearly every other 855 ever made.
Deere has done an excellent job of making sure there's an upgrade path for every component, so you will never be left high and dry with no ability to fix the thing. But sometimes the replacement of one part will require the replacement of other parts around that one part, to make it compatible with the later design. In one case, I broke a $400 or $600 steering knuckle casting, and because that casting was only made for the 1986 model year, my only repair path was to buy a whole new set of castings for the entire front axle, something like $2000. If I'd owned a 1987 or later 855, then I'd have been able to just replace the one $400-$600 casting.
So use the tractor while you have it, but I'd definitely be on the look-out for a later model year 855 to trade up, if you like that machine. It is a very good tractor for an assortment of "large yard" chores, I will admit, I liked mine a lot. Mine was 4wd (MFWD), and so I used it for snow plowing and many other chores.
What
@TCowner explains sounds very cool, but I've honestly never seen that. The two 855's on which I worked (mine and a good friend's) both had only the original mower control. And yes, your 3-point hitch will go up and down with the mower deck, that's by design. You can tie the hitch up if you want, but most don't bother. Do put a bungie or spring between the lower 3-pt hitch arms, to keep them from rubbing against your tire lugs, but otherwise don't worry about it.
And as I described in a post above, you can lock the mower deck up by raising it and then turning the height stop wheel (by your left ankle) to highest setting. This will prevent the mower deck from dropping when you lower the 3-point hitch to use an aerator or other implement. But there's no way to lower the mower without also lowering the 3-pt, which means no leaving an implement on the tractor while mowing... unless you want the implement on the ground.
One huge advantage you have is that there were a
lot of 855's made and sold. So if you think you're missing parts, you're probably within 10 or 20 minute's drive of several others, probably on various dealer lots. My local Deere dealer
always has a few of these sitting in their used sale lot or maintenance/repaire queue, which I can drive over and look at if I'm wondering what's missing on my own.