I am in a similar situation - I am planning to start something with a few of my friends. And this is something we are trying to solve too. The biggest issue with your proposed solution of using Linux and in-house servers is maintenance. Remember we don't have a dedicated IT person, one of us will be handling that as additional (hopefully minor) responsibility with the major responsibility being software development. So, having a server, keeping it up, maintaining it with updates etc seems like a lot of work, especially when we're just 5 people and are working without any salaries (so may be using a very small office and we may not even have an A/C). We are inclining towards hosted solutions and are close to signing up with Google Apps. The only other option I found is Zoho, but given our size (4 or 5 people), Google Apps is free and we're thinking of taking it, at least initially.
My only concern is when we grow, and want to migrate out of Google Apps, how hard is it going to be? We are assuming it should be possible since gmail supports IMAP and all of docs can be taken offline. If I want to migrate it to say Exchange or Thunderbird for mail and Libre Office or MS Office for other apps later, I'm hoping I should be able to do it.