Are these your only choices? Rust repair including paint matching and getting the AC working on the 240D will be costly unless you can do that work yourself. More than the $2000 you expect the 300E to cost. Can you pay the money up front for a car that has what you need with fewer compromises?

Here’s a W124 like the 300E but with a Diesel engine - https://cincinnati.craigslist.org/cto/d/93-mercedes-benz-turbo-diesel/6435426328.html

Here’s a 240D MT in a less attention grabbing color but might need less work - https://cincinnati.craigslist.org/cto/d/1979-mercedes-240d/6430551629.html

Here’s a sharp looking 300SD - https://cincinnati.craigslist.org/ctd/d/1985-mercedes-benz-300-series/6428169263.html

Sixto
98 E320s sedan and wagon