Used record prices are inflated, as is grading. There are some specialty dealers who are known for consistent grading but they aren’t cheap. It really depends on what kind of music you are chasing. For new issues, the Tone Poets are great fun and Chad’s uMe/Verve series has some of the great warhorses for relatively little money. Quality control isn’t bad on those but there are still issues-- it’s the nature of the thing.
As to buying in "lots," you may be forced to travel if it is your intention to buy collections from others but you’ll be competing with dealers. Record shows can be a good source- if you know what you are looking for- kind of depends on your location and how far you are willing to travel. There are collector groups that gather in certain cities--you need to find an "in" with somebody to get you started in those circles-
I’ve bought collections, but I knew the prior owners and most of that was years ago- I overpaid partly due to friendship but it wasn’t a big deal given how much pricing has gone up on some things.
Some of the standard issue hard rock can still be gotten cheaply on places like Discogs but you are vulnerable to misgrading- if hard rock is your thing, I’d recommend that you use the Hoffman forum and do some deep searches for comparisons of pressings- there are huge differences in sonics depending on mastering, country of manufacture, pressing plant and vintage.
Entering the LP market now would require you to do a deep dive. I know one dealer who acts as a "concierge"-- people will give him a budget, he knows their taste and will chase stuff down -- all at a price. That’s the more esoteric side, and costly.
I tend to agree with those who say think about this carefully before you dive in. Even if you have top notch LP playback/front end, your real expenditure is going to be the records. I would not want to try to replace what I have in today’s market- not only b/c of price but because a lot of it isn’t there. (There will probably be a "die off"-- that’s usually where you see dealers at shows with a whole pile of the "usual suspects"--another audiophile went to that big listening room in the sky).
Not to be a buzzkill, but.... :)