New player wanting help with EDH

So I just recently got into Magic and I have a few normal decks together that seem to be working well. I borrowed a friend's EDH deck and played a 4-way game that was loads of fun, so I now want to see if I can make my own for this format. Problem is that I have no idea how to really build an EDH deck and would like some advice. Here are my options for my commanders.

Ob Nixilis, Unshackled: Seems like a decent choice, can get pretty huge if I put a bunch of kill cards in.

Arcanis the Omnipotent: I run this guy in my green/blue bounce deck, the draw effect would be incredible but his self-bounce would just put him back in the commander zone.

Exava, Rakdos Blood Witch: Don't think this is the best choice, I don't own too much good red stuff and the deck would be quite challenging to build around her.

Kothophed, Soul Hoarder: Seems pretty good for similar reasons as Ob Nixilis, probably a solid choice.

Tasigur, the Golden Fang: Not sure if this guy is even legal in commander, as his casting cost only includes black but his activated effect can be paid with either blue or green. If he's legal, he'd probably be a good option, but I don't know if I can even run him as my commander.

Dwynen, Gilt-Leaf Daen: Seems amazing, but I only own 15 Elves I could legally run in a deck with her as the commander, so this may not be the best choice.

I alwo own Kiora, the Crashing Wave planeswalker, but I didn't know if those were legal as commanders. The deck I borrowed had a planeswalker commander but it specifically said he could be the commander, so is this even legal? Probably a good bet if I can run her as she gives me access to 2 colors.

Any other general advice on constructing an EDH deck would be greatly appreciated, thanks in advance for any help!