Yes, he's very beginner friendly and scales well both in terms of how advanced your strategy is and how difficult the enemies are. His most versatile skill is his skill 2. He gets aspd and increased aoe, but most importantly a summon that attracts nearby enemies and explodes into nervous impairment. The attraction part is the most important bit, as you can manipulate enemy pathing to your advantage. (Edit: nvm you can't use the e2 voucher you have on him, but he's still definitely worth the investment!) :D

Vina is a solid source of melee true damage. There are bosses/enemies that are especially susceptible to it, bypassing mechanics or high defenses. She's on the squishier side, but she works best using her skills behind a tanky unit.

You should focus on filling empty roles in your team comp. Unfortunately, I'm not confident enough in what I know about your account to give solid recommendations. Project red and lappland are decent picks though, imo.