[Question?] Must have spell/trap cards for the base on most generic decks? Best way to buy them?

Just got back into the game recently. Got a collection of about 500 or so cards but the high majority of them are monsters and most of the magic cards are like set pieces that only really work with a deck based on that specific set.

I'm wondering if there is like a common knowledge amongst experienced players of magic cards that should always be in your collection? Stuff like Mirror force, Negate Attack, Monster Reborn and/or Polymerization.

And to follow up that question, what is the best way to obtain these cards? Is there a convenient structure deck / set that comes with a few of these cards? Or if I need to buy them as singles where is the best place to find them for a good price? (Was looking at mirror force on amazon earlier and all of them were like £3-4+ just for 1 copy, not sure if that's just what I would be looking at for a card like that or if amazon isn't the best place for singles)