I would recommend that your do the get the following from theme.
- There host. It is good to know a persons host before starting work on there website.
- You should make sure that you have full admin access, or if they have multisite then Super Admin. This way you can have all the freedom to make the changes.
- FTP/cpanel access. They don’t need to give you access to there host login, but you should make sure that you always have access to there FTP before starting any work on there website.
- Lastly I would make sure that you make a full backup of there entire website (MySQL database, public_html, etc.) before starting any work at all on there website.
If you have access to the FTP, and admin then you should be good.
As for localhost I would recommend using MAMP. It is more lightweight then other ones like XAMPP and it is super easy to install and set up. If you are going to make a lot of changes then you should defiantly use a localhost website and transfer the website over. To do this I would checking out this article. If you do all your changes on your localhost then it ensures that you don’t cause any errors or down time on the website.