To call Naruto [Sage of the Six Paths] as Ashura is like...
Right now Rinne Saske price is - Unlock: 3360 Fortune Coins [15960 - 26544 Coupons/Ingots].
It's luck based, but price is usually in the middle. Normal Rinne saske price is 21-26k. I would hunt such events where you can get him bellow 20k, also you get side items for free. Sometimes he is in "Lucky Stars Wheel" event for 19-21k, but event it self is a * and not worth to invest. If you won't go for this event, you should wait for fuku deals, since he is pretty often in events anyway.
But I don't recommend going for him. Can't think of any worse p1 ninja. Maybe to use him as p2 in some lineups..

Edo Sarutobi still woks. He can be a support later on. Was about to write AG Konan, but overall Sarutobi is better. Although they are both power ninjas. JMinato + 5K Madara work against anything. (tho it's ~30k coups overall).
If you afraid you won't have enough power to compete with others with ninjas you want, just wait for power fuku deals. Will get insta power boost there.
Well, Kushina SB will cost around 27-30k. It's to get her and get one skill. She may have skill trail, i personally didn't looked into it. You may want to find that out.

I recommend to watch as much fights as possible. (space time, videos, others) You may find some lineups that will * to you.