I will reply again without the link to GP,
FLU farm, you are not taking the fun out of anything. I am having it delivered to my house in the city, so I will not be driving it anywhere except to the backyard. I will eventually make it street legal, but that is low priority. After I get it reconditioned I will take it out to the property I have in South Central Colorado. I figure the time I save in having it close by, and access to tools and parts will make it worth not having fun with it till later. As you suggested, about the first thing I plan to do is replace the fluids, belts and hoses.
Welcome. Given the history of the GP auctions, folks early on tended to get the better units (not all the time) while recent auctions tended to be units in less good shape. This is not a hard and fast rule but an observation. I realize right now you are in the mode of having just bought that scratch off lottery ticket and expecting you win the prize but I will try to reinject a dose of reality.
If the major components are not 100% you may be subjecting yourself to an expensive run down the rabbit hole. Luckily the underlying parts are typically low mileage very beefy Mercedes parts but abuse by an operator or bad maintenance can still damage expensive stuff. The good news is that parts are available with some sleuthing but the bad news is that if you need major parts they are expensive A transmission is probably a 6 to 8K investment. Your goals should to be ignoring the minor systems and seeing if the major equipment is in good enough condition that's its worth dealing with the small stuff.
At least one member on this site got a "bad one" with trashed transmission and obviously it had been picked for parts. I think he recently got a good one and the old one is now his private parts source.
Definitely a full lubricant change out and investigation of what come out of the drains is step one. If the rig will start and run, you may want to hold off on messing with the fuel system (for now). If the air pressure comes up great but if it doesn't, it may be worth hooking up a temporary supply from another source. One trick I realized recently is you can cut your air system diagnosis time somewhat by pushing in the emergency air system valve (large round button on the center console) this cuts out the trailer braking system that you can deal with later. Once you have air, drain the air tanks multiple times or even better remove them and the drain valve and clean them out. Its almost a guarantee that the air system is crapped up due to poor design and many of the fancy systems only work with adequate air delivered to the them.
Now assuming its running try out the transmission. The SEE transmissions reportedly has weak synchros so shift slowly and carefully. Now try the 4wd, you may hear a lot of air leaks, don't worry about them for now but you will need to deal with them later. If you have functioning 4WD, congratulations you may have runner instead of potential parts vehicle. Note we all are probably lusting after parts when someone declares a parts truck
. Note the diff locks may not work but there is good chance they can be freed up.
With respect to dash, lets hope the rats didn't have field day. If when you get it running, you notice electrical accessories and gages not working, the "rat" has done its damage and you will soon have choice, either keep it for an off road beast hoping that the damaged wiring does short or find a quiet spot and plan on 40 to 80 hours of patching the harness back together.
I would definitely take a look at the track bars and exhaust since GP apparently is known to damage them when moving the SEEs with an all terrain forklift. There are sources for used track bars and an exhaust shop can probably patch the exhaust.
Good Luck