SaaS customer interaction management

Hi all, The company I am working for is planning to release our cloud based software to the public.

All client interaction has happened via e-mail until now, which was ok, since we had only about 20 SaaS clients, plus a few dozen managed clients.

Is there a "best-practices" for managing thousands of SaaS clients?

Features would need are: customer support, ability to offer customized upsells, marketing to people whom we only have their e-mail addresses yet theyre not members, ability to attach billing info with our backend communications system.

After doing some research it seems like our solution will be something like: export user info into zendesk and salesforce by API. Zendesk for help, Salesforce for upselling existing users (i think) and tracking billing/account info, and mailchimp for sending mass mails to existing users to non-users.

Is there something that I am missing? Thanks, HN community.