what to look for on a used 1980 300CD
 

I'm going to be checking out a locally offered 1980 300CD soon.

it's listed as an auto, running strong, using WVO for several years (in the summer only, this is in virginia) with ~ 250,000 miles

from my research it looks like
-non-turbo 5 cylinder diesel, mileage is high but it should have a while left before it needs a rebuild if it has been taken care of
-2 door w123 chassis
-a/c doesn't work, common problem on a vehicle of this age?
-WVO use could cause problems if some seals and lines haven't been switched from rubber to synthetic

since you folks are very knowledgeable about these kinds of cars, my question is:

What problems should I be looking for before I buy?

I know that rust is a major issue in these vehicles, should I be looking anywhere besides around the gas cap and at the lift points?

What are signs that the engine/transmission are on the way out? A slipping transmission would be obvious, but is there anything else specific to look/listen for?

I'm a little worried because the listed price ($1500) seems quite low for a vehicle in the described good condition. Pictures seem good, original paint with minor rust visible.

I understand that a vehicle this old is going to have some problems so I have set aside another $1000 for some immediate repairs. I can perform some of the maintenance myself, (oil, brake/tranny fluid, brakes, minor rust repair). I, like everyone else, don't want to purchase something that's going to suck away my money and time and leave me at a bus stop in a couple months.

Thanks.