I help maintain a page[0] that keeps a list of the best performing routers that support OpenWRT and DD-WRT. It allows to sort by Value, Performance or Price.
Just make sure, what batch of C7s are you buying from.
TPLink started to lock down the firmware, due to the new regulation about locking down wifi devices. So if you get an unlocked C7, you are fine, if locked, you get to keep their firmware on the device.
Currently, the only safe choices for OpenWRT are Linksys WRT1900ACS and Turris Omnia. Both are a bit pricier.
[0] http://rooftopbazaar.com/routerfirmware/