Thanks, folks. Yuz guys have sized up the situation well for me. This is disappointing, but at least I can say I'm wiser now than when I got the M-B dzl bug seven years ago and bought this dog.
Although it is a slushbox, it is a very clean car, but not pristine (like the beige CA 240D featured in The Star a few months back). When I said it has very little rust, what I meant is that there is no visible rust anywhere. I only see some on the inside of the fuel door and the inside sheet metal of the driver's door.
Nonetheless it sounds like it's not worth it to fix it if I'm selling, even if it only needs the valve job.

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Now you're suckin' Diesel!

'88 300SEL
'92 300D 2.5 Turbo
'96 Lincoln Town Car. The Jazzmobile.