Correct wp-content ownership and permissions

By default, all files added by the Apache itself will be owned by the server’s user, in your case www-data. But since your users use FTP to deploy, those files are owned by them, and it makes the Apache scream for your attention. Changing the ownership of wp-content folder to www-data isn’t insecure at all, as this is a default user used only by the Apache, but as you said, it is only a temporary solution.

For a permanent solution, I’d recommend Tom‘s solution from this post on ServerFault:

Attempting to expand on @Zoredache’s answer, as I give this a go
myself:

• Create a new group (www-pub) and add the users to that group

  groupadd www-pub 
  usermod -a -G www-pub usera    # must use -a to append to existing groups
  usermod -a -G www-pub userb
  groups usera    ## display groups for user
  

• Change the ownership of everything under /var/www to root:www-pub

  chown -R root:www-pub /var/www    # -R for recursive
  

• Change the permissions of all the folders to 2775

   chmod 2775 /var/www 
  

  2=set group id, 7=rwx for owner (root), 7=rwx for group (www-pub), 5=rx for world (including apache www-data
  user)

  Set group ID (SETGID) bit (2) causes the group (www-pub) to be copied to all new files/folders created in that folder. Other
  options are SETUID (4) to copy the user id, and STICKY (1) which I
  think lets only the owner delete files.

  There’s a -R recursive option, but that won’t discriminate between files and folders, so you have to use find, like so:

  find /var/www -type d -exec chmod 2775 {} +
  

• Change all the files to 0664

  find /var/www -type f -exec chmod 0664 {} +
  

• Change the umask for your users to 0002

  The umask controls the default file creation permissions, 0002 means files will have 664 and directories 775. Setting this (by
  editing the umask line at the bottom of /etc/profile in my case)
  means files created by one user will be writable by other users in the
  www-group without needing to chmod them.

Test all this by creating a file and directory and verifying the
owner, group and permissions with ls -l.

Note: You’ll need to logout/in for changes to your groups to take
effect!

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EDIT:
If you are serious about running a hosting server for different clients, perhaps consider setting up and running your PHP differently then it is by default (as mod_php). I’d recommend running it as PHP-FPM service, as it can be tweaked to use the user’s permissions, which will further prevent PHP server to have a full ownership and rights over literally all files from the web root.

This is an excellent article explaining different ways for running PHP. I am not an expert on the matter, and you can get more help from the ServerFault.