Hi
A) MainStage 2 System requirements: Note these increased with later versions of MS2
MainStage 2 - Technical Specifications
Minimum requirements to install all applications
- Mac computer with a 1.25GHz or faster PowerPC G4 processor (PowerPC G5, Intel Core Duo, or Intel Xeon processor highly recommended)
- 1GB of RAM (2GB or more highly recommended)
- Display with 1024-by-768 resolution (1280-by-800 or higher recommended)
- Mac OS X v10.4.9 or later
- QuickTime 7.2 or later
- DVD drive for installation
- PCI Express, ExpressCard/34, USB, or FireWire based audio interface recommended
Additional recommendations for MainStage
- At least 2GB of RAM for large EXS instruments
- USB musical keyboard, control surface, or guitar pedal board (or suitable MIDI control device and interface)
- Standard graphics card in any Mac Pro, MacBook Pro, iMac with Intel Core Duo, Power Mac G5, or iMac G5
NOTE: If you do not already own MS2 you will not be able to buy it from the AppStore
B) MS 3 System:
Minimum System Requirements
• 4GB RAM
• Display with 1280-by-768 resolution or higher
• OS X v10.9.5 or later
• Requires 64-bit Audio Units plug-ins
• 6GB available disk space for minimum install / 35GB hard disk for full Sound Library installation
https://itunes.apple.com/gb/app/mainstage-3/id634159523?mt=12
Your best bet could be so get an external drive, install OS X 10.9.5 or OSX10.10 just to run MS3, keeping your existing OS for backwards compatibility
CCT