From your post it looks like:
- You are not a local who can speak and read Cantonese and can talk to repair places
- The cost of a phone isn't trivial to you
- You are very unfamiliar with phones and can't do internet research past posting a shotgun blast of questions on a new forum.
Combining all three, you should seriously reconsider whether buying a used phone off some random stranger is a good idea. If anything goes wrong, you are basically completely out of luck with no way of redress. e.g. you run into the proven Apple battery slowdown problem, tough luck.
Rather than buying in to a 'status' brand and taking on a whole load of risk, consider buying new (for warranty) from a brand with a good reputation. Xiaomi has a great reputation for value and function here. Same with the Moto series. There are a bunch of other brands and models that have been recommended before on this forum.
You should only be buying secondhand if you are familiar with the product, you are familiar with the environment you're buying it in (you didn't know that you're living in the middle of the secondhand electronics market!), and you can afford to take the 100% loss if the product just breaks on you.
Edited to add: I'm not even sure what people charge for secondhand iPhones these days so don't know your budget, but OnePlus could be a good option, they have a 2 week return policy if you don't like the phone.