16GB wifi model still running strong and snappy on Aosp 6.0.1, DC kernel, ParrotMod and super su 2.71. Everything is smooth apart some graphical bugs in games
Refund it and get 2013 version instead. 2012 version got problem with emmc as result its really slow and crashes alot. If you still go with it well then flash Omnirom https://forum.xda-developers.com/nexus-7/development/rom-omnirom-6-0-1-homemake-t3282311 , parrotmod, supersu 2.65 and pico...
Yes you will lose key. Nothing will change after unlocking bootloader, every app will still be here and working, camera will remain same. Just follow official https://developer.sonymobile.com/unlockbootloader/unlock-yourboot-loader/ link