Make sure the climate control/heater & air condition controls,work.
Make sure the vacume door locks work.
Make sure it shifts evenly.
Is the seats and head liner in good shape.
Now you question. Milage.
I have two 84 and 85 both turbo diesel.
One has 325,000, and one has 200,000 miles.
These will go 500,000 easy and up to 1,000,000.
Three things.
Change the oil hot and at 3,000 unless you use synthetic.
Adjust the valves every 15,000. Check chain for stretch.
Change the chain 150,000 to 200,000.
Also I change my oil filter every oil change.
I change my fuel filter ever other change.
I am a big beleiver in preventive maintance.
There are other on this board that feel this is a waste of money.
But this person feel's that anyone driving a CD is stupid. But this 6'6" fellows loves those big doors. And hates being stuck on the road.
Charlie