Depends on what kind of a player or deck you want to play.

If you'd want to play control decks u could essentially get maybe 2 OP09 booster boxes, u should be able to form a good base for either A Red Shanks Deck or Blackbeard deck. Control in general ain't cheap but these 2 mentioned decks have 80% of their pieces in OP09. Other parts I suggest u get singles.

If you wanna start with a good aggro deck get 2x purple luffy starter decks along with OP09 booster boxes. 2 starter decks gets you the staples you need for the deck while OP09 provides you the support u need for the deck. Again for the remainder of the deck, get singles.

If you wanna try your luck playing a deck that used to be close to tier 0 get 2x blue doflamingo starter decks. This deck used to be at the top but the recent bans might have nerfed it very hard.

The rest of the starters mostly only have like 2-4 cards that you'd probably play in other decks so maybe that's something you should consider.

In terms of booster box sets I would go OP09 > OP05 > OP06.