I wouldn't advise spending for seals. Ive bought maybe 4 or 5 since Ive been playing but everything has been ac*ulated through events and dailies. The best way to do it is just to save the ingots for a spending event if you do choose to spend them on seals. Then if you spend for seals you just hold them for the next rebate and you get a 20k spending reward for doing so.

As far as Hidan goes. He is worth the value. He makes time space a lot easier and dominates. I'm not sure when they will pair another beneficial spending event with him but I would say to wait unless you absolutely need him now. I'm always about getting the best value for your dollar. I topped up during top ups and then will splurge it during spending event.

As Ilijime stated though. Roshi isnt a must have now. The best main for him is earth atm. If you're light main then you essentially need Hidan for it to flow well to get Roshi's 30 hit chase off lol.

Ultimately. From the 200 seals you buy, you would probably only ever use Roshi in the long run and he will become a lot better once he receives his buffs. Yagura, bee, and Yugito are fun but, they're more like arena ninjas and maybe ranked. Spending for Hidan will give you better use over the course of the game regardless of meta he c*ways dominate. You may find yourself using him more often aswell. I personally would spend for Hidan if I had to choose between seals or him. I would just make sure theres a solid spending event if you do decide to get him.