You can do all this on a single Linode. You will just need to configure your mail server (e.g., Postfix) and web server (e.g., Apache or nginx) to handle multiple domains. The Linode
library can help you with that.
Based on the
stated system requirements, Quam Plures should have no trouble running. You'll probably have to install it yourself, as it doesn't look like it's packaged for many (any?) distributions.
I'd recommend against installing an FTP server unless you have special circumstances that call for one. SSHD includes the ability to transfer files by SFTP, which is much more secure.
You will have to maintain things yourself; Linode does not manage the software on your virtual server. If you have prior experience, this should not be a huge problem, though you may have to re-learn some things. The Linode library, distribution- or application-specific help forums and mailing lists, and this forum are all resources you can use to get assistance.
Debian stable and Ubuntu LTS are both good choices because of the large amount of pre-packaged software and the length of time they continue to be supported with updates. CentOS also has a long support lifetime, but includes far fewer packages in the base repository. In a lot of ways, it's a matter of personal preference.