Survival is one shop you don't have to worry about, it's the easiest to get points for by far. I have 5* Suigetsu/Kabuto and 4*Tobi/Lee, Tobi last 80 frags will come at lvl 78~ from story and few days away from 5* Lee and i even bought cultivation runes few times because was impatient (pro tip, don't do this lol). So just buy whatever makes you strongest at this moment. Which if you plan on being water, Suigetsu is very nice for 9 tails and Tobi is best f2p ninja in current patch.

It will be very long time till they remove any, first they add new ones, then discount old ones and only then they will get removed if they will at all. At least that's how it went on all other versions of game, so i think ours will be the same. The next shop update should add some missing Akatsuki members if i remember right and none will be removed.