More than likely you can only get the v2. You could also look at the asus rt-n53 it is about the same price and also runs dd-wrt. It supports guest ssid with default firmware you would have to load dd-wrt to get ipv6 support.
On the 802.11ac...how many of your machines have AC adapters in them. Just like running a mix of G and N unless you have all AC adapters you will unlikely get the full advantage of buying it. 802.11ac is a year away before any router can claim to meet the standard...it is not set yet. This runs the risk of incompatibility much like the "pre-n" stuff had. So far there is no 3rd party firmware for any of the chipsets that run AC so you are stuck with what the vendor offers.
I keep reading reviews, and most of them are good, but when I do come across something bad about it, it's always about signal strength for the WiFi. People say it's crap, and if you are in a 2 story house or anything like that, it don't work.
I wish there was a way to test something like this.
All routers will have a issue with a 2 story house. The antennas are designed to send the signals parallel to the floor. To get them to go vertical you need to turn the antenna 90 degrees which is not possible if they are not external.
The vast majority of routers transmit at the maximum legal power. It really hard to tell if one router really is better than another since people houses vary so much. You will good and bad reviews for almost every router there is.
If both your PC and router can do 2x2 or 3x3 mimo (it means the both have 2 or 3 antenna) you tend to get better signal,not because it stronger but because the devices have the ability to use the reflections to they advantage. Not sure if that helps if there is a solid floor between.
You could try a direction antenna that would concentrate the signal into a narrower beam and point it though the floor I guess. But again you need the ability to run external antenna.
The ASUS N66u has external antenna and support almost every feature you can think of and I think it also has IPv6 support. It also runs the largest DD-WRT images. It is a little expensive though $160. Not sure if it will would any better than another router though a floor though