1. You can start developing using 3.8 now. The upgrade path should be well documented. Of course, if there is a feature you really need you could wait, or start developing using 3.9 rc1 which has been released.
I am fairly sure there is an option in the setup wizard, although it's a while since I've run the wizard. Worst case scenario, you'd have to do a manual install without using the wizard:
