The dealer will be able to run a vehicle master inquiry that will show all the recall / warranty work on the vehicle. My wiring harness replacement showed up on this.
It can also be visually inspected if you're not sure.
If it hasn't been done, I'd have them do it and pay full price for the car. It'll cost them $850 to replace it parts and labor. I don't think $13,500 is that much money for a 95 E320 with approximately 100,000 miles.
As far as the evaporator, if the a/c works fine, I think it'd be hard to get them to go down in price just because it 'might' go out and because it's expensive to replace.
Head gasket leaks are common and easy to notice.
I'm surprised you think 100,000 miles is too many miles for a Mercedes. In my opinion, it's just broken in.
Last, the M104 engine was used until 1999 in the S320. I have to believe that Mercedes itself must have thought this was a great engine because they used it in their most expensive chasis. I think it's a fantastic engine that is smooth and powerful, and the only fault the engine itself does have is oil leaking at the head gasket. There haven't been any complaints about its refinement, power, durability, longevity, or efficiency. It puts out a lot more power than the M103 engine, which I think you'll also appreciate.
If a dealer is selling it, they must've thought enough of the car to sell it on their lot. Cars that are too old, have too many miles, or too many problems end up at the auction.