It's a good engine, inline-6. Has a common problem of oil leaking from timing cover. Not a big deal.
$6k sounds a bit on the high side. It better be in great shape for that price. Make sure you take it to a Mercedes mechanic and have it looked at first.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino