What kind of hosting do I need?

There are two options depending on your capabilities and how you use the builder.

First option

It requires a WHM/cPanel or DirectAdmin account at the reseller level. With this configuration the builder will connect to these panels via api to create and manage domains, sites and mailboxes. See how builder is using communication with WHM/DirectAdmin

Second option

The second option is to use a regular shared hosting or server without connecting it to the control panel. In this configuration, users will not have the ability to manage mail accounts. In order to automatically connect subdomains, wildcard DNS should be enabled and assigned to the root directory. Adding custom domains is possible by manually adding them as an alias of the main domain using hosting control panel or having a dedicated ip address and assigning it to the public directory.

Both options will allow you to add new external servers in the future if they will have WHM or DirectAdmin panels. See also server requirements.