2 new 65series batteries are a lot cheaper than a diesel starter.
I'm a truck/coach mechanic and only thing i'll put a used battery into is a gas powered car. don't screw around with used batteries, it will cost you more in the end.
did you mean your voltage is 11.7 when running?
after you start the truck it will take a little bit before the glow plug system shuts down completely, check your voltage after running for 5 min, will give you a better idea.
here in Ontario, I can get BXT 65- 950 cca batts for just over $100 each.
cheapie "NEW" starter for an idi 7.3 is over $350
my 7.3 powerstroke clubwagon had a weak starter in it when I got it at 360,000km. with good batteries in it and clean connections, the starter is still hanging in there at 405,000 km, only had to go under and tap it once
summer: RIP 72 Bronc, 68 Merc,
Daily:94 Exp xlt , Barney the PSD CW
projects: all on hold for now