Buying a car ...
Consider the list "threw away 10,000 on strippers", "gambled away 10,000 at the casino", "spent 10,000 on a car".
The first two are fine: (A) they are one-time expenses and (B) at least you have a good memory. But "spent 10,000 on a car" is financial insanity. Because the costs are ongoing, monthly, extremely substantial, and never-ending.
Buying a car is financial madness.
Selling stocks to buy a car is a really unfortunate idea.
It's hard to imagine anything more negative than selling stocks to buy a car.
Buying a house...
Money put in to buying a car is pretty much the single most sickening waste of money possible. In contrast money put in to buying a house is, in almost all cases, an entirely good thing. So, great.
Of a group of stocks, which to sell: tax implications
Regarding the question "of a group of stocks, which to sell", in many jurisdictions there are tax differences between your stocks A, B, C, D due to issues such as length of time held.
If this is the case, you have the answer.
Of a group of stocks, which to sell: trading decision
Unfortunately, deciding which to sell of A, B, C, D is exactly the same as deciding which stock to buy of a group of stocks.
It is a straight-up stock picking exercise.
You literally have to time travel to the future and decide which one will go-up-least in the future.
As you have noted,
"Non Answers - I googled around and there seems to be a plethora of advice..."
Any advice on which stock to sell - or buy - from a group A, B, C, D is: bullshit. Total bullshit.
The final, unequivocal answer is: you must guess which one will go-up-least in the future, and that's it. Any other advice is as valueless as stock-tip newsletters unfortunately.
Regarding measures such as "volatility". If "volatility", or anything else, was a predictor of the future price of a stock - everyone would be a trillionaire. Nothing, at all, has predictive power of the future price of a stock; there is no stock picking.
One possible logical approach
People often ask "which stock to buy?" The only answer is "buy an index fund, because, obviously, it's completely impossible to stock pick."
Similarly, some would say that: if you have to cash out from stocks A, B, C, D. The only possible way to do this is to tell the future and know which one will go-up-least and sell that one. Hence, since stock picking is comic, the best thing to do is "sell the basket" .. equally sell from A, B, C, D.
In this way you minimize the risk of losses.
Speaking of an S&P index...
If one currently owns stocks A, B, C, D. It's better to sell them all and buy a plain broad index fund. One could take this opportunity to dump all of A B C D, take away the relevant cash for the house, strippers or gambling (forget the car: never sell stocks for a money-waste), and put the rest of the money in an index fund.