I’m assuming since it was installed by a legitimate shop, they would have looked everything over first, changed filter and fluids though.
I don't think you want to "assume" anything. Contact the shop that did the transmission work for more details. Replacing an LR4 transmission usually involves taking the body off the frame. To go through all that labor and drop in a used transmission with 110K miles on it seems like an intentional shortcut. Knowing a rebuilt transmission was just installed would make me more confident in the purchase over some random transmission. I "assume" owner #3 got rid of the LR4 because he/she knew the transmission was failing.