Wait for official answer since my information may be out dated..

As per cPanel system requirement, you need at-least 2GB RAM hence with DO, any plan above USD 20 is to start with.. Actual system requirement may depend on your number of resell account and their load on server..

You do need license but AFAIK, DO doesn’t provide that so you have to purchase it separately.. There are many re seller, you can get one at around USD 15 a month (per VPS)..

As per as the cPanel system requirement, one should select the Latest release of Cent OS in x64..

cPanel is a paid software, and yes it comes with everything you need.. One should install cPanel in a clean VPS deployment, cPanel will take care of rest of the things.. Since cPanel is paid software, so they have a really good support forum and off course, you get their customer support too.. Read more here -> http://cpanel.net/

Thank you very much hunky. As a matter of fact I stumbled upon an Open-Source CPanel and it worked on Ubuntu on Digital Ocean. I am going to make a tutorial here regarding the steps of Installation when DO starts accepting tutorials back.

Hello, Wade

You can also install cPanel/WHM via our Marketplace. All you need to do is to click the install button and the droplet will be quickly deployed.

New installations of cPanel come with a 15 day free trial. For further use beyond that point a valid license will need to be purchased directly through cPanel. https://store.cpanel.net/view/cpanel/license-options?type=VPS

Please refer to below link for further details:

https://marketplace.digitalocean.com/apps/cpanel-whm

Hope this helps!
Regards,
Alex