Basically, yes, the customer will be limited to buy one of the CMS packages I will provide.
They will only have access to the CMS backend and webmail, no hosting control panel.
Don't tell me you have never seen this business model (Wix)? ;-)
Softaculous requires a paid license to use it for custom installs. I am not averse to paying for anything, just don't need the extra layer of 3rd-party software.