Why concentrate on rare champion? Because that is fare worst than Epic or Legendary ones. Maybe Khael is worth, but compare my list. I just really few rare champion raised to lvl60. I also started with Kael, and I get Ninja. But my first rare to lvl 60 vas Coldheart.
Only 4 lvl 60 character was: Coldheart, Pain Keeper, Aphotechary, Curpulent Cadaver ... but wait for a proper legos for endless shield team.
Do not waiste time for rares. You have great legos.
Respectfully, I don't see what this has to do with OP at all. He doesn't have your quality of legos, nor gear. Would love to get him using Kymar at some point but that's going to take gear and books.
So we're focusing on a team that includes one rare (the one you mentioned, even), and furthermore there are many epics/legendaries that would be worse investments for buggs relative to the best rares.
I would also say Coldheart being the first rare to 60 is going to be a massive mistake for most players beginning in the game given that she can't campaign farm, has fairly high stat requirements, doesn't help us progress in CB, is going to be worse in Dragon.