In the past I was always buying an Android phone first and check if CyanogenMod runs on it second. Several Android phones later and after holding back on older 4.x.x versions by countless manufacturers, I have learned that if an Android phone doesn't run CyanogenMod it's kind of worthless (from a hacker point of view).
These days I always check if an Android phone runs the latest version of CyanogenMod first and buy it second. My current phone is a Galaxy S2 (i9100) which according to CyanogenMod stats[1] is the second most popular phone. It runs Android 4.3 and you can update it nightly right from inside the settings. (Settings -> About -> CyanogenMod updates)
These days I always check if an Android phone runs the latest version of CyanogenMod first and buy it second. My current phone is a Galaxy S2 (i9100) which according to CyanogenMod stats[1] is the second most popular phone. It runs Android 4.3 and you can update it nightly right from inside the settings. (Settings -> About -> CyanogenMod updates)
[1] http://stats.cyanogenmod.com/