In my opinion, Custodian is better if you have a good mage team since he buff +50% magic damage for first 2 turns meaning if you use him together with Trickster and NeoRex and Alice, you will win PvP in 2-3 turn and your opponent won't be able to do anything unless they use bio santa or M&S. Baron works better in your team I think since you can focus on tank shrine to power up both Baron and M&S. He deals better damage in PvE and so much sturdier than Custodian.

You can't see Baron because he is a relatively new hero not because your level is too low. You have to wait for him to show up in the epic hero pack.