BUT the car in question is 20 years old...the paint will not be "concours", the door panels will
be sagging, the carpet/dash/gauges/vinyl will be faded a bit or otherwise show some amount
of wear...

The ONLY thing this car has going for it is it's low mileage, but considering that 20 years
is 20 years, it's going to need the same amount of attention that a well cared for 150ker
would require, in order to bring it up to "show" quality.

Here's my thought:

Say you find a clean 123, in let's say Arizona or Vegas...and not somewhere coastal like NJ.

It has 80k miles, been well serviced/garaged, and it's white, original paint is in great condition.
(Being that white paint is probably going to stand up to 20 years better than anything metallic,
this is a plus for the sake of "originality".)

The asking price is a little high for a 123, say $6800, and even though the NADA says $4200 and
Kelly says about $2800, you talk the seller down to a slightly less outrageous $6200 and go for it,
being somewhat of a vintage diesel fanatic, and also recognizing the "true worth" of these vehicles no
matter what crappy 'ol Kelly says.

The car is reeeeally nice, but like any older car, it's getting a bit tired around the edges. You decide
you want it "perfect", or as close as humanly possible.

You replace the wood, dash, carpetting and front seat covers, overhaul anything worn in the
steering/suspension, along with any other "little" things the car may need.

Let's say this costs about $7500.

You've now spent less than 15k, and have a NICER car, both cosmetically and mechanically, than
what home-slice on ebay wants for 20k. For 20k to be an option, it would have to have more like
10k miles, in addition to being flawless otherwise. By this I mean the factory wax coating still all over
the engine bay, and not a gallon of Armor-All.

See what I mean yet??

Just for reference, 1984 300D:

NADA says between $3750 and $7100

Kelly says about $3000