To add to what has already been said:
While you are leveling up (in exp parties), focus on having the main avatars (Fenrir, Titan, Levi, Garuda, Shiva, Ramuh). It's not terribly important to have the harder to obtain avatars (Fenrir, Diabolos, Odin, Alex). Diabolos can certainly be useful if you are in a party of black mages and want to just stick his refresh aura up (although that is a REALLY boring way to play), and Fenrir has decent DD, but the other avatars will suffice for exp.
If you want to do any endgame, though, you will need at least Fenrir, Diabolos, and Alex (as well as all of the other avatars you should have already had). Fenrir and Diabolos have certain avatar-specific bloodpacts that are required in events like Voidwatch. Alex has the ability to give your party perfect defense, so his usefulness goes without saying. Odin is the only avatar I can see really skipping out on and not missing much, but I'm sure he's got a niche use somewhere.
As for race, I'm a mithra SMN, and I play several other mage jobs as well. Once you hit 80+ and have abyssea gear, your mp pool won't matter. This is somewhat true for most jobs now, but especially true for SMN, in my experience, because we are so limited on how often we can spend mp anyways. I don't remember the last time, even outside of abyssea, that I truly ran out of mp. (RDM or SCH subjob will contribute to this, too. /WHM is only the best from 1-70). So really, choose whatever race you think looks best.
~SMN99 BRD99 BLM99 BLU99 WHM99 DNC99 THF99 RDM95 COR95 SCH95 RNG75