Just to add my thoughts. I started a very similar project only with three Moodle instances:
- Moodle 3.51 - live
- Moodle 3.51 - test
- Moodle 3.6 - Development
I threw in a Mahara instance and an Owncloud instance.
See below for our resource specification. The OS is Linux Debian. Also for what it's worth, we have a separte Linux Ubuntu server running Learning Locker for analytics.