I think you could make a pretty decent starter CB team with:

• Brogni (block debuffs, ATK up, passive healing, hp burn)
• Deliana (leech) - turn off A2 and A3
• Fahrakin the Fat (ally attack, def down, hp burn, poison)
• Frozen Banshee (poison)
• [Anyone] - put whoever you want here for now, but what you really need is someone with DEF up and ATK down. There are plenty of options, look for one ideally with a 3-turn CD, 2-turn duration DEF up, and either an ATK down on the A1 or on a 3-turn CD, 2-turn duration move.

For dungeon teams, I think that Coldheart and Frozen Banshee are both pretty solid options.

Given the above, I'd say your next 60 should be either Deliana, Khatun, FB, or Coldheart. Kaiden is a solid choice too, as you'll typically want a reviver, at least for early teams.

In Rector vs. Coldheart, I'd go with Rector. Other than Fire Knight you won't be getting full value out of Coldheart for awhile. Rector on the other hand would fill a few roles as a reviver, healer and giving overall team protection. 

However, with Brogni being 60'd already, I also don't mind going with someone like Frozen Banshee to help out with farming Dragon and going into Clan Boss (which kram is bang on about, you have a really nice core to start). 

I think I'd go Frozen Banshee next, then possibly Deliana.

Thanks!!

for me frozen banshee is a must to lvl to 6* as she is your main poison in cb an ddragon until u get better posioner

or Apothecary. will help in a lot of areas