I have a neighbor who has a 680 which has been sitting in a shop for years. It appears to have significant fluid leaks - perhaps mainly hydraulics. I'm considering trying to get it started and working. Im wondering what steps I should take to try to ensure I don't cause damage trying to get it back up. Somewhat obviously I need oil and coolant to try to get the engine up - do I need to worry about the hydraulic systems to just see if I can get the thing running? If I did get it to run - what would I need to do or worry about before trying to use it or move it etc..? although I have performed lots of car/truck repairs over the decades - Ive never worked on heavy machinery nor hydrolic systems - so don't assume I know those basics.. thanks!
Welcome to HEF, small farm ! I would change the motor oil and filter and look for any coolant that may be in the bottom of the oil pan. Check the radiator and hydraulic fluid levels. If you are experienced with diesels, drain the fuel tank and change the filters. Loosen the fuel lines at the injectors and bleed out the old fuel. Get a couple of cage bolts for the brake chambers. You may need them if the brake system won't build air pressure. It is good insurance to have them. Eventually you may need them.
Might help if you don't have any books on it to go here and click on "Find Parts By Model Number" do a search on 680 and scroll down to see if you can find the one you are looking at: https://www.rpmmachinery.com/case-ce-parts.php It is just a parts book but can be helpful. Also if you can post the serial number it would help others know exactly what you have and make suggestions more accurate.
And to answer your next to last question, if you do start it with no hydraulic oil in the tank, you will destroy the pump, which probably costs more than the machine is worth at this moment.
All 680s require a minimum of 16 psi of air pressure to force oil into the hydraulic pump. The air is plumbed directly into the hydraulic oil tank.
I see mention of checking engine oil for coolant but I would also be at least loosening all drain plugs to see if there is water in there. Even if it's been sitting inside there could be significant moisture in various compartments from condensation. And if you are not familiar with the machine I would strongly suggest getting hold of a copy of the operators manual be sure you are checking things the right way and know what kind of fluids each compartment takes. Hopefully your neighbor has at least that much information for you. Or maybe see if there is a dealer who will let you see a copy? There seems to be several available on Ebay and such, but just be careful that any you might buy is correct for the S/N of the machine you are working on!
When you add the new fuel it may not hurt to add a can of sea foam to it .