Hi guys, I'm not sure if this is the right place to post about this - sorry in advance.
I received my Zenfone 5 today (I'm in the UK) from a UK eBay seller. I was surprised to see the packaging was all in Chinese. I started downloading my apps back from the play store and noticed I kept getting permission prompts (allow/decline access) about apps being downloaded. This makes some apps unusable - as soon as I open Whatsapp this prompt keeps popping up and wont go away every time I click allow or decline. I've also noticed that SuperSU is on the phone so I assume I was not the first person to use the phone.
Anyway, I excitedly performed a system update search and was told there are no updates available. This is the current software build number:
ASUS_T00F_WW_user_9.17.40.16_20190811_0963
release-keys
010175_201901130121
Also says the last update was done on 01-01-1970.
Confused by this, I downloaded the latest SKU WW kitkat update from the Asus website and tried to sideload it into my phone using adb. I got this error: "signature verification failed".
I really don't know what to do now guys. I just want to enjoy the phone and update to kitkat, I tried my best

Hopefully you guys will understand this more than me - I've exhausted myself google searching for help the past few hours. Thanks in advance for any help!