As to your mental health, I am unqualified to respond. As to a diesel that has not run in 7 years, I think it is salvageable if it is a decent car.
If the owner just started it, I'm sure you will have him start it for you before you buy it. It sounds reasonable to me that he could do what he said, but I would bet that a 7 year old battery didn't take a charge and then start the car without some help.
Plan on replacing the battery, belts, hoses, oil and filter change, fuel filters and air filter at a minimum. Also definitely change ATF and filter. Also plan on treating the fuel tank for algae, a fuel tank drain and strainer change wouldn't hurt.
I gave my son my 240D when he turned 16. When he went to college he left it behind. It sat almost two years before I started it again. It was amazing how easily it ran after getting the starter to spin.
I would go through a few short cycle oil changes after putting it on the road. A couple of hot oil changes in about 500 or 1,000 mile intervals.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in