Hello Paul,
There will be spamassassin on the shared
VPS Hosting but that would be configurable. You can configure it as per your requirements and you will need to configure it individually if you setup separate cpanel account for different domains.
If you do things from control panel of one account then it wont become server wide change. You will need to do same thing for all accounts under your reseller account.
Fantastico and all other features would be as it is. If you need full control of the
VPS Hosting and full access to space and bandwidth then you can consider full
VPS Hosting rather than shared as the price wont increase much. Additional advantage that you will get with full
VPS Hosting is the ability to create accounts with unlimited space and bandwidth. As you are feeling comfortable with WHM cpanel so now you can make potential benefit from it once you get root access with the WHM.