I have located an 81 TDT wagon about 250 miles north of where I live. It is said to look good, with new paint (ivory) with a sunroof and MB-Tex upholstery in good shape. It has 177K miles, and comes with no records. I don't think it has a third seat.

The owner does not seem to be an expert on mechanics. He says that one of the rear schocks is broken, the driver's side door does not lock, and the hydraulic strut that holds up the back gate has lost its gas and needs replacing. He thinks that there should be a self-levelling suspension gizmo under the hood, but says he can't find it. I am not sure what to look for.

He bought it from a dealer last years and since bought an 87 TDT that he likes better.

The asking price is $3125, which is the price given by Edmunds for this car sold by a dealer in excellent shape.

I would not call this better than "good".

As I have been busy grading end-of semester tests and the like, I won't be able to have a look at it for a week or so.

Any and all knowledgeable comments will be graciously apreciated. Thanks.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)

Of course you will certainly know whether the asking price is fair once you have a chance to drive and inspect the car yourself...
To put in perspective I paid $4500 basically sight unseen for our '81 300TD wagon because of the records and receipts available for all work done since '88. Most of this I was assured was at an MB dealership. It was well worth it in my opinion as the car is a quality vehicle with zero mechanical flaws and just a few little things which have required attention.
This model will have the self-levelling rear suspension. It can indeed be expensive to repair or replace depending on your amount of DIY. The reservoir for this hydraulic unit is under the hood, to the far left lower corner as you are facing the engine. The unit can cease to function for many reasons: sometimes it's just low fluid, sometimes it is simply adjustment of an "arm" which actually affects the levelling action on the rear axle, sometimes the equipment really is broken and need replacing: call Phil at Fast Lane and price the air cells and/or shocks. To my knowlege you cannot convert this car to "regular" rear shocks.
The tailgate shocks are relatively easily DIY.
The driver's door is mechanical, basically, but via a vacuum actuator attached it in turn opens and closes all other locks in the car~ it seems strange that the driver's door would not work but all the others do?
How about rust? If the car is near the gulf or the ocean it may have some rust happening, albeit not to the extent of the snow belt vehicles we see up here!
If the seller is not terribly knowledgeable on MB's then that may be an advantage to you in negotiating. I'm sure I'm missing something here but it's a start at least.
So much is relative, you know. There are people out there who will tell you don't even look at the car, it's way overpriced. Then there are people who wouldn't mind dropping 10 or 11k on a really pristine wagon with the right receipts and paperwork, you almost have to decide what's right for you, you know?
Sorry for the long-winded post and good luck!

what happened with the car?

Thanks for asking. I did take a look at it.
I don't think it is actually worth the asking price.
Either I would have to pay someone else a ton to fix it, or I would have to spend a lot of time writhing about on the patio fixing it myself

Here is my description of it.

Advice about 1981 300TDT I might buy

__________________
Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)