Sounds fun! I should warn you that what you're asking for here is something that is not objectively simple, and that there is no one correct path. So let me give you an overview of how I think I would do it, at least my first iteration of it.
- Purchase WHMCS - https://www.whmcs.com/
- Purchase DO reseller addon for WHMCS - https://www.modulesgarden.com/products/whmcs/digitalocean-droplets
- Create user-data script that is run on all new server provisions.
User-data script must:
- Update base OS
- Install Minecraft server
- Install MC server control panel and/or plugins
- Add a script to the server that regularly runs OS updates
Now that user-data script has no "right way" to be written. I would treat it like a bash script because that's what I like to do, and you can put a bash script in the user-data. Note that the DO reseller addon for WHMCS states the feature to support this:
"User Data - Execute Custom Cloud-Config Or Bash Script To Customize Server On First Boot"
As far as what it actually looks like to configure that within WHMCS, I couldn't say for sure. Maybe ModulesGarden would let you try it first. Never hurts to reach out to them: https://www.modulesgarden.com/support
I hope that at least gives you an idea of the direction you might go in for this :)