I have tried to build a similar app 3 years ago. I used twitter bootstrap, but unfortunately failed. Bootstrap was the most flexible since the average (open source) CRM solutions have too many features to build an app that meets your requirements for a MVP. You could consider the Appexhange framework from SalesForce to directly push it to a broader audience, but then you have the dependancy of building on an external API. Good luck i truly believe you are on something valuable!