Great, Thank you!Turning on FileVault 2 should have no impact whatsoever on the life of your SSD. As far as a performance hit goes, it's so very slight that you won't notice it. Just make sure you do not forget your FileVault 2 key. Register it with Apple using your Apple ID. ;D
The shop did.Who formatted the disk and what format did they use? In order to use FileVault 2 the disk must be formatted as Mac OS Extended Journaled. Any other format, or non Journaled, FileVault 2 can not be turned on.