so obviously rust is a big deal... even small bubbles on the rear wheel wells, door sills, rear gate... all of these will get ugly fast.
the rear main will most likely leak. just a question of how much.
i would pay attention to the knuckles. take a look to see if there is chunky gunk on them or more oily gunk. if its more oily i would say the axles seals are blown. you should prolly plan on a rebuild anyway.
overheating... that is a big issue. check the radiator to see how old it is, check the overflow bottle to see what the fluid looks like... drive it on the highway then in traffic, see if it heats up... check the operation of the fan clutch.
then, the head gasket, obvious but give it a good looksie...
thats all i can think of