Head gaskets can be bad if the engine was overheated. If there is a whiteish/yellowish gell on the inside of the oil filler cap or on the oil dip stick then it's bad. The other head gasket symptom takes longer to see --losing water from the fill tank.
And the same things you would look for in an SD. Windows all work, ac, ventilation system pods, clunk with foot on brake while shifting from D to R, blow by smoke out oil filler, blue smoke out exhaust, worn front tires indicating loose ball joints / steering parts, sun roof works.
If all good then it's a steal at $1K.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K