Let me answer your questions in the order you asked
1. DirectAdmin looks and feels just like cPanel when you use the Icon-grid theme, which we use. Most of our customers have never faced any issues with DirectAdmin and it has all the features that cPanel has and the process to set things up in cpanel and directadmin are relatively same. If you were not a power user and if not for the orange theme becoming blue, most users can't say a difference between cPanel and DirectAdmin (icons grid) because they both look so similar.
2. Our daily backups are stored on external offshore storage, and our daily backups do not account towards disk usage. However, if you take backup manually in DirectAdmin using the backup option, it will be stored on the disk. However, unless you want a copy for personal safety, there is no reason why our daily backups with 30 day retention wouldn't work.
3. Yes, it has one click installer to install WordPress and other related scripts called "Softaculous" - the standard in cPanel servers too.
I hope I answered your questions. Do let me know if you have any more questions or doubts too - I would be more than happy to help you out!