I sure wish the OP would drop on by with an update.
We recently purchased a 1978 Mercedes 240D. 4 speed, no A/C, with electric windows and sunroof.
It is a 2 owner since new car and has just over 107,000 miles. Interior looks like new and it runs great.
The original owners ordered it and the n did a European delivery to pick it up. Back in 1978 that was something.
Since they lived in rural Oregon and diesel back then wasn't easily available, they had installed an auxiliary fuel tank in the trunk.
I haven't filled it yet but it looks to be about 15 or so gallons.
That, added to the 18 gallon stock tank should give me over 1000 miles of range.
While I don't plan to make this one into an expedition vehicle, it's just too nice.
I would like to see what the OP has been doing with one that has been made to explore.