My client is nervous with no GUI on the server. Are there drawbacks to installing something like XFCE or Gnome? Are there better desktop environment options for a server setup? Alternatively, I don't know how to explain to the client that the GUI is a bit useless fin this case. Techies with customer service experience please advise.
The only drawbacks are that your server is now running the extra overhead of a GUI. Nothing to be nervous about, unless your server is very short on RAM ;)
XFCE is a decently lightweight choice (I'd certainly advise against KDE, anyway) but any desktop environment won't directly interfere with the function of the server.
If your client doesn't get it though, you, uh, probably have a good deal of trouble with this client in your future... Good luck!
If Ubuntu server is already installed and GUI is mandatory, just run
Depending on what they need to do, they might get away using Webmin