Yes, it is a self-selecting group here, fortunately anyone who owns and cares for one of these in the year 2020 is likely to be doing so by choice rather than coincidence.
The cluster looks original as far as I can tell. I don't see anything to suggest it has been replaced. The diesels didn't come from the factory with the boost or volt gauges like the turbo gas cars did. A few may have had additional gauges added as dealer accessories, but most look just like this.
I see a couple of cracks in the dashboard top near the middle. Pretty much all of them do this (same as with the 240 series), even ones that live their lives mostly in garages. This one looks pretty minor as far as they go.
On some of your questions:
Parts: not quite as simple as it used to be, but good for a 35 year old engine and car. The D24 and D24T engines have some lucky things that help in this regard: they are close relatives of the common 4 cylinder and 5 cylinder VW/Audi diesel and gas engines and most parts are shared, and thus inexpensive and widely available. The D24/T engines were also used in commercial and military applications into the 2000s and the parts support for those helps as well. The Volvo car still has good parts availability too, this basic platform was produced with few changes until the late '90s and the cars tend to live long lives so there are many thousands still in normal use.
Service: you are right that the chances of finding a neighborhood shop that has the right equipment and knowledge for serious engine work are low. But for regular basic maintenance (fluids, filters, etc) it's nothing out of the ordinary and any good mechanic will be capable. For in-depth work like timing belts, water pumps, fuel injection system work, etc, you will want to bring the car to someone who knows them well. Folks who can help with this exist in your part of the country, fortunately, including Tom Bryant in Maine and others.
Some dealers might surprisingly still be willing and capable to get work done too, the odd handful even today seem to still have all the tools for the old models and employ someone who remembers how to use them, though no doubt it would not be cheap. You could ask about this at the dealer that is selling this car, since they claim to have done all the service to it, and here it is still evidently alive after 35 years of care from them.
Price: no way to really know, as you say the market is small enough that "book value" doesn't mean much. These cars are classics now and sometimes you see nice examples sell for surprisingly high prices, $10k or more. On the other end, one in poor condition is worth nothing. In between, it depends on the particular car, who's looking for one, and what they want to pay. Right now I imagine this dealer is hurting and should be happy to listen to any fair offer you propose. I think if they would go for $3-4k you'd get a neat classic car for a good deal, and the dealer would hopefully be smart enough to take it and be happy. But who knows?
They mention full service records. Do you know what those show for timing belt replacements, wiring harness replacement, routine service history etc? You may want to ask for whatever they have and see what you can learn, as those questions play into what the car is currently worth and what it may need soon.