CRM for small not-for-profit organisation

I volunteer for a small not-for-profit organisation and have offered to research or CRM options. Use cases: 1. Pull all communications (email, social, notes from phone calls) with a contact into one place. We use Mail Chimp for email atm and have FB, Twitter and Instagram channels. 2. Ability to report on success of campaigns; either growth of funders network or increase in one of donations as a result of an email/post.

Also, we want to keep costs down as far as possible and the solution would ideally allow up to 10 concurrent users.

All suggestions welcome!