No, the Mason WordPress theme is not required for an ITS hosted WordPress website, but it is strongly recommended as it meets brand, technical, and accessibility requirements and is the only theme we will support with any in depth troubleshooting/assistance. More information about theme support is here.
Generally, yes. There are a few administrative plugins that we require, and some Mason-specific plugins that we pre-load onto our WordPress websites. There are some third-party plugins that we recommend. There are very few that are prohibited. Other than that you are free to use whatever plugins you feel will help you meet your business needs. We also have tips on how to select appropriate third-party plugins.
Yes you can, but it is a good idea to let us know when you purchase a paid plugin and provide the account and login information so that we can be prepared to help if necessary. More information about using paid plugins is here.