[raulus]

This isn’t much in terms of advice, but since no one else has mentioned it:

Buy what you like! Whatever will bring you joy and make you a little bit happier every time you see it or think about having it in your collection.

In my humble opinion, that should be your primary focus when buying cards, particularly nice ones that cost a lot of money.

If you’re hoping to make money on it, then that’s just a nice side effect, and icing on the cake. But also somewhat unpredictable, even with the excellent suggestions and hopes and dreams that others have suggested here. You might make some money on it, and you might not. But your personal enjoyment should be something that you can predict with much more confidence.