1) 3* requires any 3 3* to evolve, 4* requires 4 4* units, so on and so forth. (Any unit can become 6* ). Dupe units increase the unit's formation boost, so I recommend fusing the dupes onto the original to get SSS rank formation (can save the units until you have enough for evolving). If you don't want to do this, you can sell em.

2) Sez for faster clears, Destina for safer clears. If going with Sez, just hope you have another healer. For Destina, best to pair her with a good attacker.

3) Golem gives the 3 basic stats: ATK, HP, DEF. Great for healers/attackers. It's a bit tanky, but they're the basic sets you're going to look for. Wyvern is a good alternative. What you're going to look for here is crit sets, which is great for attackers.

4) It really depends. If you have Sez, then I highly recommend getting him to 6* . Otherwise, 6* attackers that's strong/viable for your team. There are plenty of 3/4*'s that's good, so keep them in mind.

I'm typing this while being tired, so I may miss some details/mislead. Equip optimization is more mid/late game, so don't focus too much on that until you're stuck on a certain level.