Like Maxbumpo ssid, it looks like it is has the 'evil servo' which can be problematic.

Some areas to inspect for rust: rocker panels (rust through under the sills), floor pans, in trunk under spare and in rear fender pockets), lower front and rear windshields, and beneath the "evil servo".

Don't forget to inspect the condition of the transmission and power steering fluids, steering box play and leaks, date codes on tires,flex plates, exhaust hangers, rear differential mount and shift rod bushings.

$5k is high in view of paint condition, mileage(??), tired interior, visible rust (there is always more), and especially if the AC is not up to snuff.
Ask if regularly driven, currently resistered, back registration due, salvage title???

Realistic price woud be $3,500 (+/- $500) or less if AC "just needs refrigerant".
In the end it's worth what you are willing to pay to acquire it and also bring it up to your standards for road worthiness.

Bring cash in hand, if you are definitiely interested.
Be polite, describe the deficiencies you find, make your offer with the cash on hand - it is a good motivator for closing a deal that the seller may not otherwise accept.

I paid full asking price on my 78 300SD because the R12 AC was functional.
They handle amazingly well for their size and weight.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022