I went through a similar process in 2006, when used vinyl was far cheaper than it is today. If you are talking classic rock, one of the best sources for info on better sounding pressings is the Hoffman forum- but you have to dig- many will be older postings, by members who have done the comparisons. While dead wax alone is not altogether determinative (the inscriptions on the play out space after the last groove of music), it is instructive.
In terms of sources, I used Discogs a lot simply b/c what I've been buying in the last decade or more is not something you'd find at a local store- early heavy rock from the UK; so-called spiritual jazz (typically US pressings in the '70s during a low point in vinyl manufacture).
Jazz- Tone Poets and Ume/Verve series are good and relatively cheap. One of my favs on Tone Poet is Katanga! which is currently out of print but hopefully, for those who don't have it, they will do another run.
I developed a taste for oddball stuff- thus, Abdul Wadid's By Myself was reissued (with some help from a DSD copy of the tape) and reissued for 30 bucks. An OG costs 4 figures, so there really isn't much choice, unless you have unlimited funds.
EIL was a go to for me for a while- they aren't cheap but grade conservatively. Good source for English rock from the Island pink label era.
If you are buying used, from a platform like Discogs, enter into a dialog with the seller -- their response will give you better insight into how careful they are on grading and minimize grief. I've had very few sour transactions that way.
I posted about Woody Shaw's Blackstone Legacy here a week or so ago- not "rare" but hard to find a clean copy- took me a few years. And now, even more expensive. Craft is reissuing, allegedly from the tape, cut by K. Gray and it isn't expensive.
In some cases, the reissue or a later issue may make sense only because the desirable copies are now so expensive. When I was buying up Dark Sides, A3/B3s were commonly 50 bucks a pop for M-. I'm sure they are 5X that now.
Do your due diligence both in terms of sonics of particular pressing and the seller.
Sorry I did not make this shorter.