https://www.reddit.com/r/openwrt/comments/423rn6/best_2015_o...
https://www.reddit.com/r/openwrt/comments/36s530/better_ac_r...
Initially I was also planning to buy the Archer C7 but it seems OpenWRT is not flawless on it. This is unfortunate since that appears to be the recommended router for OpenWRT, so there is nothing on which OpenWRT works perfectly.
That was the situation 10 months ago. I'm not sure if things have changed since then. If they did, the OpenWRT has not been updated: https://wiki.openwrt.org/toh/tp-link/tl-wdr7500 According to some recentish posts on their forum there may be hope: https://forum.openwrt.org/viewtopic.php?pid=342710#p342710
I wonder if it isn't better to just go DIY ( http://arstechnica.co.uk/gadgets/2016/09/diy-homebrew-router... ) with pfSense or with the PCEngines apu1d4 and use a separate switch and access point.
Right now, it is very important to have a router with firmware you can trust and upgrade, otherwise sooner or later it will join a botnet. Therefore for me any router option without FLOSS firmware support is eliminated.
Edit: Just read the comment above about OpenWRT being dead. Looking into LEDE.