Playing the OP scenarios at home

I'm going to buy the Season of the Shackles scenarios for my group to play at home. Is there anything the scenarios that necessitates we used the OP playstyle of everyone having their own class decks? Or can we all pick a character and build decks from the S&S set like normal?

You can do either. If you don't already have class decks and you're not planning on reporting the sessions, then just build the characters out of the box.

Only if you are planning on reporting the characters as PFS ACG characters that you need to purchase class decks.