I've been pleased with my 300SEL. It has the same engine as your 300E, only more room to work on it.
$10k should get you any W126 SEL except for a later 560SEL cream puff. A 300SEL shouldn't be any more than $5k or $6k in pristine condition. I got mine for $2900. It needs new seat covers and I've put about $300-$400 into the engine. The exterior is outstanding though.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino