A few thoughts to add
I didn't know clear coat could be damaged because you lived near a beach.... sounds more like he didn't want to pay $$$$ for the proper paint. However, if it looks good, that's up to you.
A/C may still be ozone-not-friendly R-12 Freon, $ just to fill, $$ if leaking or parts need to be replaced.
A metallic clanging sound may be loose/broken air filter bracket, common weakness on OM617. See if the air filter housing is loose, watch it and listen to it while engine is running, try to stop the noise by holding/moving the filter housing. If this is the source of the noise, it's annoying and a relatively easy/cheap fix. It could also be 'nailing' which sounds like it says, someone trying to hammer a nail into the block. It's a side effect of diesel compression ignition, and to some extent normal. Too much can mean injector or other problems.
Slow takeoff may be that you're not used to a diesel, or could mean it's slow even for a diesel. Could just mean a few vacuum/pressure lines need cleaning.
Black oil in a diesel is common, even new oil turns black in just about the time it takes to drive around the block.
Slushy oil, and not enough of it, could mean Previous Owner (PO) is not inclined to regular, or even irregular maintenance, and the oil is way past its due date for an oil and filter change. It may not have damaged the engine, but it's not a good sign.
Overall, I'd offer no more than $1,000 given the A/C and alignment issues, along what appears to be a bit of engine compartment neglect. It might require $1-$3k to address those issues, and then you'd have paid up to the $4,000 that he was asking.
Best regards,
Jim