"Hi there, I've recently purchased a used MBA with the previous owner's iLife suite. I want to do a fresh install, and was wondering how I can keep the previous iLife suite ?"
What I would do:
1. Clone the contents of the hard drive to an external drive, using CarbonCopyCloner
2. Boot from the cloned drive
3. Re-initialize the internal drive using Disk Utility
4. Install a completely fresh system onto the internal drive
5. Boot from the fresh system and at the appropriate moment, choose to migrate accounts and apps from the external clone.
6. Be aware that Migration Assistant doesn't give you the option to pick and choose amongst migrated apps. You may have to delete some unwanted apps after the migration has taken place.