At first, you will take a good project machine (to learn) but either luck up or buy a bomb.
Then you notice things you do like,and don't. Standard flipper arrangement, no spread, split or unnecessary extras.
You quickly figure out if the bells or chimes need help, especially the bells (they may need dampening).
So you shop for another,as well as Research here and everywhere else. Strong opinions about Back Glass art begin to sway you,
"Vulgar, Satanic, Too Quaint, Not related to the Play/Theme, Offensive racial themes, depictions of smoking or prostitution, stereotypical caricatures, you name it.
So you ask yourself, "Can I tolerate seeing this lit up" The desirable Themes are unobtainable/ not for sale.
You move on to Play Field features, quickly realizing drop targets, free ball gates and return kickers are a good thing.
So the machines with the great art and lacking these features are out of the equation. Then you experience an EM Spinner and realize a machine with a combination of these features would be the ultimate.
So your quest seeks the Best and learns to ignore the rest on Craigslist and other sales sources.
You find that others have figured all of this out Millenia before you and have all the good machines and even the bad ones(for parts)
A new appreciation for the Machines you have sets in, the Watchful Eye still occasionally perusing the Craigslist and Classified offerings.
You re-paint the Cabinet to your second machine.....To be continued

What you said. The younger Crowd is getting educated as well. They have little or no memories or connections to EMs but the word is getting out, if you like to play pinball and don't have a lot of money to invest there are great deals. They are easier to fix, the parts are available, and most importantly no cheesy electronic sounds from the early SS.
The Market for EMs has left the barn, oh yeah, its rising. These machines will soon receive their due, break away from artificial price constraints and clean working examples will sell for markedly higher prices.
Sounded good,didn't it?
Youtube kinda ruins it, you find a cool SS and get excited, then you see it in play. Millennials don't like the primitive sounds, they are ok with me since I played them when they came out.
EM has the "Campy Edge" with original Themes and a lot of cool Artwork.
High voltage and fast play are part and parcel without dropping 200-400 on circuit boards.
Bottom line the EM Buyer becomes a Snob, Aficionado, whatever you wish to call it. A lot of "Fringe" games turned down but higher prices for the good ones.