Listing factors that affect value is the easy part.

Assessing how they fit together is the hard part.

It isn't the simple mathematical equation the NADA guide suggests, where you to add $X for this or that option, deduct $X for that color, pick the adjustment from the mileage chart, and stick the number on it.

These aren't Honda Civics, where there are hundreds of thousands of cars, and hundreds of thousands of buyers -- a situation that allows to to price cars like commodities.

At this point, every one of these cars is effectively unique, with enough distinction in options, history, and condition to it very difficult, if not impossible, to make value judgments any narrower than a 10-20% range. The more unusual to color, options or condition, the harder it becomes -- because there aren't that many comparables.

What is someone willing to pay for a 87 coupe with 75k miles, in special order Summer yellow, with all documents, records, tools? Who knows because it is not even clear such a car exists. What if it is Prussian Blue with no sunroof? Or factory paint-to-sample in Signal Orange?

Stepping away from impact era 911s, the vast majority of which are not true "collector" cars, here are some factors, in order of precedence, that make collector cars inherently valuable:

Race History (in that specific car)
Homologation
Rarity (as a make/model)
Special Editions (performance oriented)
Rarity and Desirability of options (color, body style, sunroof, appearance packages, etc.)
Originality
Mileage

These factors are baked in, and you can't really change them. The last factor of originality only matters to a point. All original, "preservation class" cars will trump factory correct 100 pt restorations -- but that is only at the very top of the market. A tatty original car will be worth less than a super high quality restoration. Same with mileage -- in a preserved car with verified low mileage, it can be a big factor. A total restoration effectively resets mileage to zero anyway. And in most classic cars, it is totally unverifiable anyway.

This brings you to:

Provenance and Condition. Provenance is mainly a matter of confirming the above factors, and confirming they are genuine.

Condition is "king" -- but it also the variable that is most easily (though not necessarily inexpensively) influenced. You can make any car condition 1 if you throw enough money at it.