If you want someone who's not gonna be powercreeped soon/outdated soon, I would not recommend Kushina [Habanero]. She's good, sure, but I feel like her era is coming to an end. Her frags aren't in events as much anymore, and when they are, they are in "lower tier" redeem events (eg. Naruto's ramen, alongside other less relevant ninjas). This usually happens when a ninja's relevance is declining.

Edo Minato is good, but some coming ninjas that are on the release horizon will be better at countering him than those in the meta right now. When GNW Madara gets his BT, he and Rinnesasuke will do better against Edo Minato. BT Kimono Shisui should also be a match for him, since he causes unmissable damage on mystery and standard.

Honestly, if you only have 15k coupons, I wouldn't start using them to buy Minato or Kushina (or really, anyone's) frags right now. Wait until you have 30k coupons and see what the meta looks like at that stage. That will probably be like 4 months down the line, and things may be different by then.