I wouldnt do coldheart. She is gear intensive and needs books because otherwise she has really low hp. I also wouldnt use CH for CB unless its easy and even then you would be better off just upping an outlawmonk if you are.doing easy CB.
Warmaiden is excellent for most dungeons and arena and she is f2p friendly because she drops on the deadlands stage of the campaign so you can book her up easily. She also does the big version of defense down 60% on her aoe which is better then some legendaries.
High Khatun can give you speed boost and turn meter manipulation for arena and dungeons.
Apotthecary does the speed boost much better and heals. Arguably the best rare in the game.
Screech (bat thing) Also give turn meter boost plus some other stuff. Im not familiar with his kit but he is used in high level arena.
Ultimate Galek is great for spiders with his 2 aoe and one being a hp burn.
Cardinal is also an excellent toon as well.
Zelotah is a good addition because he gives a shield with healing and does aoe with sleep debuff.
Marked is another excellent rare that you can use along with the void hound.
The problem is epics take alot of potions to ascend so where possible utilize rares to help you grow.
Erinyes is actually pretty solid since her buff.
Again I would not be focusing on CB right now. You need to be putting together a solid team to start doing dungeon grind and getting 5 solid players to 60.
Go to youtube and look up stewgaming and watch his videos. He looks for rares and even uncommons that can be used in dungeons as high as 20. He also explains why a coldheart isnt the killer that you hear about. His champion guides outlines everything including their masteries. Chosen and HellHades do this as well. Also if you have an epic and not sure if its good just do a search on youtube with the name and raid and usually you can find out if a champ is good.
Food for sure is maeve, oathbound, shaman and those off top off my head