I don't play much standard these days, but Mutavault is a versatile card that can function as a win condition in a deck like Esper which has relatively few of those as well as operate defensively when needed. Being that you'll be playing the deck just until rotation, you should be fine running without. In fact, you'll get a boost of better mana since you're playing 3 colors and don't have to run a colorless land. It's a lesser benefit than having a land to attack and block with, but it's something.
Mutavault is a good modern card as well, but only fits into a few archetypes. It isn't nearly as ubiquitous there as in current standard. Demand will drop for them when rotation comes. That said, the price has already fallen a bit in the last few months and I'd be surprised if it completely flat-lined. They will probably go down to $12-15 from the current $20-25.
Sphinx's Revelation is an unfortunate card to have to buy into, because it is a pretty essential component of your deck and these will also lose a lot of value at rotation. Elspeth is safe. She's likely to be a finisher in control decks of some kind and maintain value for the next year You'll be able to trade them for something else if you decide you're through with Esper.
You can still use the M14 version. As long as the card is legal in some form, you're good!
