It depends a lot on your budget and target consumers.

• Someone purchases WordPress hosting from my website (doesn't exist yet)
• Either a Droplet is created under my DO account, or a virtual container is created on one of the droplets under my account (either with Docker or Vagrant)

If you plan on selling hosting publicly try a module that integrates with popular billing platforms.

WHMCS has one here - https://www.hypnotic-monkey.com/whmcs-plugins/digital-ocean-whmcs-module/
Blesta - http://www.blesta.com/forums/index.php?/topic/3505-digitalocean-module/

• The installation script is then ran to install LEMP, WordPress, set up the MySQL database etc.

Based on how you price a server, you can either use a paid control panel like cPanel, Plesk, InterWorx etc or a free panel like ISPConfig.

Another option I like is ServerPilot, a hosted control panel service. It has very less features compared to the ones above but is very easy to use.

• The new WordPress installation is available on an IP address - which can also be pointed at with a domain name, once the DNS configuration has been completed by the end-user

I guess this would need some manual coding to integrate properly.

Ideally, I would like the droplet to be added to some sort of server monitoring service so I can get notifications of anything that goes wrong

For a cheap monitoring service try Uptime Robot. Pingdom is another service with all bells and whistles.

And if you offer managed WordPress hosting use ManageWP to update and manage your clients' WP installations.

A new hosting company will attract a lot of spammers and fraudsters so use a service like MaxMind and FraudRecord to filter them out.

you can resell digitalocean server directly from your whmcs using this module. visit https://oddcodes.com