Hold off on the purchase for now if you can, as it will probably not be too long before Apple introduces a second generation of the 802.11ac AirPort Extreme.....likely using the "final" specifications for 802.11ac.
The WAN port on the current 802.11ac AirPort Extreme has also had more than its share of problems with speed and connection issues.
No one knows if a "draft" AirPort Extreme can be updated to provide the "final" specifications, but if history is true to form, Apple routers in the past could not be updated since this would have required a hardware change.
Luckily, the "final" 802.11n specifications matched the "draft" specifications.
Whether or not that might be the case with 802.11ac, we'll have to wait and see.