PLEASE help me save a 955K, Having starting problems

Ok, i have a family friend who has a late 60s CAT 955K parked back in the woods. It has sat for 15 years, it was inside of a building but the building has now collapsed so i would say it has been outside in the open for 10 years.
The diesel is red in color and it has a full tank, am i able to run it?
Cleaned intake out, removed filter it was junk.
Added 10 gal of hydraulic fluid but needs 20 more
Primed with clean diesel
Put 2 gal of oil in it still needs 3 more but i want to make sure it will start and run for a minute
The antifreeze is nice and green when i pulled a plug of the side of the head, at the radiator filler cap it is just water
Was left in Neutral, ran great when it was parked, exhaust was covered, undercarriage is good.

But i dont want to do anything more until i can get it to turn over and start
I wired up two 12v 650CCA batteries to make it 24v, starter clicked, clicked, then turned over about 1/4 of a rotation. click, click, then 1/4 rotation.
so i stole 2 larger 1250 CCA batteries from mine and my dads truck, same thing. but then something buzzed and it stopped trying to start with the batteries in the battery bay, hooked the batteries up together then with jumper cables ran them straight to the starter ground and solenoid. turned the key, same thing. click click but this time no rotation. Engine is free and turns over by hand, i think i dont have enough juice. all the batteries i tried were hot and ready to go they even started the cars i took them out of when i put them back. Wtf do i need to do

You'll need to do some electrical diagnosis. Using either a volt meter or find (make) a 24v test light,
reconnect the batteries, make sure all connections are clean and tight. (Using jumper cables is not likely
to make a reliable connection).

Connect the meter or test light across the starter lug (the large connection below the solenoid that goes
into the starter case) and ground the other lead to the starter case. Try the starter. If full voltage is
present, and the starter is not turning, there is a problem in the starter. May just be stuck brushes (dirt
dabbers).

If the voltage is low, and the starter and cables get hot, the starter may be shorted, or the engine too
hard to turn over (seized, rusted).

If no voltage, start moving the test leads up the line, testing for voltage UNDER LOAD. Could be bad
solenoid contacts, loose connection, bad end on a cable, etc.

Ive had similar problems on almost every tractor Ive bought. Clean every electrical connection and make sure nothing is
under load like a hydraulic pump.