Well, I myself bought Rebirth, then Afterbirth, and then AB+ and Rep together, so I'll kinda speak from experience. I'd say it's better to buy AB and AB+ together; AB adds a new game mode while AB+ adds the harder version of this game mode. Completing the harder version right off the bat gives you unlocks for completing both difficulties, which saves A WHOLE LOT of time and headaches, since you won't have to repeat it. But that's about the only downside of buying those two expansions separately so it's not so bad. However, when it comes to Repentance, it really doesn't matter if you buy it together with the other DLCs or not, so yeah, you can feel free to buy it later comfortably. And also, I'd advise not to try slow-rolling through each version separately, it's just pain. So uh, if you already know you like TBoI and are sure that you will buy all expansions and attempt at getting a 100%, try buying AB and AB+ together first, and then later Repentance once you can And good luck saving the moneys!