I have an 81 300SD and I am the first and only owner; it is my daily driver and has about 255K miles. It does not have any of the problems you identify, no rust, etc. The only two problems are that the antenna will not go down any more and the center pod which opens/closes the doors on the ac/defrost is stuck open. I have every record since 1981.

Not sure I could get more than $1.5 -2K for it. So that is a long way to say you should buy the car but not pay nearly so much for it. You might want to compare that mileage with actual records to see if everything matches.

The 300SD is a great car and easy for a mechanic (yourself) to fix.