Ask HN: Effective customer management system for SaaS startup?

We are building a business data mashup for Business intelligence for SME. We are a team of two. While the core is almost ready, we haven't done anything on the client management system so far. That includes- payment, customer management, customer support,referral etc. I have experience with Magento customization, but it seems overkill for managing only 3-4 plans and expected gradual increase of clients to at most few thousands in the near future. However, the standard security practices are most important as the clients are mostly companies.

That makes me wonder- Do people build these from scratch? What are the effective customer management systems out there to handle sell, client management, and support for SaaS? possibly open source?