Can you pay more life than you have?
No, you cannot pay more life than you have.  From the comprehensive rules (emphasis mine):


  118.4. If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)


You're not allowed to "split the cost" to pay part now and part later, either.  All costs must be payed in full before a spell or ability can be activated.CR 117.3 

In order to use Aetherflux Reservoir's ability, you have to have 50 life to pay the cost up front.