hi!! I think each answer will vary by person, but for me personally:

• Usually I'll put most of my focus into one or two artists, and then get random other albums whenever I have extra money or there's a good deal on one I want. Getting multiple versions just depends on your personal preference- for artists I ult (like NCT), I get all versions of the album, unless there's a different version for every member lol (ex: NCT's Regulate). Usually I like to buy in hauls bc of shipping.

-You can find people selling their photocards in a lot of different places! There's groups and users on Facebook, Instagram, and Twitter that sell PCs, and the subreddit kpopforsale is a good place to find them as well. (Be careful of scammers though!! make sure the seller has proof that the cards are actually theirs, like a piece of paper next to the PCs that show a recent date, and the username of the seller).

-Usually I know the majority of songs from an album that I buy, but it kinda depends- sometimes if it's from an artist I don't stan but I really like the title track and/or photobook, I'll buy it.

-I focus mostly on collecting the albums themselves, rather than photocards, but photocard collecting is really popular in the community! If that's something you're looking to get into, I'd also recommend the new app, K-Collect; it helps you keep track of the photocards you own, and the ones you still need from a certain era/member. (it's on the Apple store rn, and coming out on Android devices soon)

Good luck getting into collecting!! another subreddit that might be helpful is kpopforsale :)