I don’t know if you still need this, but to anyone who does, Kali changed the password structure from root/toor to kali/kali. This was driving me crazy as it was always root/toor before.
The way to display messages to users who log onto your server is precisely via ssh banners. These are usually stored in /etc/motd. So, since you want to automate it, you can do the following:
Make a copy of the existing file
sudo cp /etc/motd /etc/motd.orig
Manually edit the file and add your banner or, if you want this automated as well, do something ...
This is exactly what the Message Of The Day system has always been for. Indeed, a "machine will be down on Saturday" warning to users is even the example in the NetBSD and OpenBSD manuals.
Place the warning in your motd file, conventionally /etc/motd. Remove it when you restart for maintenance. Some operating systems over-egg this pudding somewhat, ...
If the root login has its shell set to nologin, instead of logging in it runs the nologin command.
Check out the manpage of nologin for more information.
As pointed out in the manpage, you can create a text file at /etc/nologin.txt to display a custom message. Otherwise it will use the default message.