When the shops get updated, more ninjas will be added in, none of the already existing ones will be removed as I know.

for group points, try to participate in as many group events as you can, joining the great war will grant you 1000 for each round if i remember correctly.

if you are almost there for karin, you should surely get her first,
or i suggest you to save your group points because konan will be added in later when you level and unlock her.

for arena and ranked battle, kurenai would be a good choice

for survival, get tobi
he works well with konan too once you unlock her
kabuto requires less frags, so you can finish getting him first before you exchange for tobi frags

tobi's given out for clearing all instance in chapter 8
and suigetsu for clearing chapter 6