May as well buy one of the fast charging ones which output 10W so it will be compatible with the 7.5W that the iPhone will need.
I plan to buy these -
RAVPower QI Wireless Charging Pad Quick Charge
Bonacell 18W Quick Charge 3.0
I just got that RAV Power unit. Can't say how it works with an iPhone because I'm waiting on the X, but it works well on my work S7.
Caveat is it ships with an extremely short USB cable (maybe 18 inches) and no wall plug.