Mercedes automobiles are notorious for running to 300k or even 400k miles, but I personally would be weary of a gasoline engined car with 257k. Perhaps if the engine and tranny have been rebuilt or replaced I would consider the car.
$5200 sounds about right for a 420SEL with such high mileage. If it were me I would probably shop around more and try to find one with under 200k.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K