This car is 30 years old; what's the fascination?

Old cars don't get senior discounts at repair shops.

$5K can buy a substantially newer car that will have done more miles but
is of this era.

It's you money.

If you have to ask YOU shouldn't be looking for trouble by yourself.
Take it to a german cat place with lots of Mercedes in the parking
lot and aks if there's anuybody that knows the 123 chassis really well
and pay them whatever they ask to do a THOROUGH evaluation inclusing
compression test (which is non-trivial in a diesel).

$5000 is a premium price and only a near-flawlesss car warrents this.

I'm always suspcious of low milage cars, I've seen too many that need
eevrything replaced because they've sat for too long. Much better is
a higher milage car that's been properly serviced its whole life. MUCH
better.