You guys clearly missed OP's question...
To answer OP, this is a great article which covers buying a used phone in general: https://recraigslist.com/2013/03/how-to-buy-a-used-cell-phone/
Generally you should:
Start by rebootin...
I still like the phone, but as you said, the slow update process and no release of kernel source code is what will keep me away from buying Huawei/Honor devices in the future. I am still on B360 despite the B380 has been released for about 2 month...
I would add laggy 60fps videos on YouTube to that list