Both Plesk and cPanel images available on DigitaIOcean MarketPlace has no minimum hardware requirements. You can choose the minimal $5 plan to start with but the performance of your VPS will largely depend on the available resources. It’s greatly dependent on the applications the site runs on, the number of visitors it receives, and the activity pattern of those users.
On DigitalOcean it’s always possible to increase the size of your Droplet! This means you can start small and monitor your usage. For example if you need more RAM at a point, you can simply resize RAM and CPU only(Reversible). If you require more disk, you can go ahead with full resize(Not possible to downgrade).
You can buy licenses at your own choice either from the official channels or a license reseller.
Based on your use case, Webhost Editon of Plesk won’t be the right fit for you. You can explore “WEB ADMIN EDITION” which allows up-to 10 domains.
If you buy a WHM license, you can create as many cPanel accounts based on your need. There are no limitations and additional licenses are not required.
Additionally you can also explore OpenSource Control Panels like: CyberPanel(Available on Marketplace), TinyCP, VestaCP etc.