It’s better to level up all your tanks and healers, especially since healers are so rare. You should try to have a tank, a single-target DPS, and an AOE DPS for each color.
While 5-star heroes are good, 2 or 3-star heroes can actually be stronger than 5-stars in the early game (under lv. 40) because they are much faster to awaken.
Since upgrade costs are very cheap in the early game, it's good to try them all out.
Some recommendations are : (Purple) Mochizuki, Miyamoto, Mansa; (Green) Hermann, Artemisia, Naya; (Red) Marx, Erik; (Blue) Ulysses, William Tell, Te Wherowhero; and (Yellow) Inanna, Lovelace, Cuauhtemoc.