But Aperture 3.3 requires version 10.7 (ML) or later, thus do I need to upgrade into ML or are there other options?
To be able to upgrade to Aperture 3.3 or later you need to upgrade to Mac OS X 10.7.5 (Lion) or Mac OS X 10.8.x (Mt. Lion). Upgrades from the AppStore will give you Mt. Lion or Aperture 3.4.2 right now.
I do not know how to find free plug-ins for Aperture 3.2.x, perhaps you can achieve the effects that you want with external editors?
If you decide to upgrade the OS and Aperture take some precautions:
- Check all other programs and device drivers that you rely on, if they are compatible with Mac OS X 10.7 or later, or if you can get upgrades. No Power PC program will run any longer, they are not supported on Lion or later. Most of my device drivers could no longer be used, and for some programs I had to buy new versions. You can get a complete list of the installed software and the versions from the "About this Mac" panel from the Apple menu or the System Profiler.
- Make a backup and if possible a bootable clone of your system as it is now, just in case you want to revert to the older system. Otherwise it will be nearly impossible to downgrade again.
- Also, in rare cases the upgrade of the Aperture library to version 3.3. caused problems. While still running Aperture 3.2.4 make sure your Aperture Library is in perfectly working condition by running all three "First Aid" options: Repairing and Rebuilding Your Aperture Library: Aperture 3 User Manual