Openbox sounds great for this. I use it personally. It's pretty simple to configure, and I wouldn't expect you to have any trouble setting it up.
If you want to place those few buttons on a panel/taskbar, I would recommend using tint2.
Depending on the text editor needs, it can just be LibreOffice Writer or something really minimal such as Leafpad/Kate.
I'd propose using Thunderbird as the email client, unless they just wanted to use the browser.
As far as the OS: Ubuntu, Linux Mint, or Debian stable, in my opinion.
Hey mate, your theme is very beautiful I really love it and I am considering getting it.
However I just have a question. How come you are using tint2 though, cause it seems like it is not even supported anymore since 2016: