Sorry worded it wrong, if you are an insider do you need to have your insider install upgraded from the original Windows 8.1 to get the final version.Nope you do not have to be an insider for the free upgrade. Check the Microsoft site for more details: http://www.microsoft.com/en-us/windows/windows-10-upgrade
I use the built-in email client, just because it is there. It comes down to what user interface you prefer - the built-in or Outlook 2016.