How did you buy GarageBand? For most posters here it helped to reinstall GarageBand.
If you have an install disk: Re: GarageBand '11 won't download additional loops
If you bought from the AppStore or if GB came preinstalled: Re: GarageBand '11 won't download additional loops
Usually you can repair permissions and repair a disk with Disk Utitility, but not the system drive.
With MacOS Lion you can do that by booting into your Recovery Partition, see: OS X: About OS X Recovery
OS X Lion or OS X Mountain Lion includes a built in set of utilities in the Recovery System. Restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears, indicating that your Mac is starting up. After the Recovery System is finished starting up, you should see a desktop with a OS X menu bar and a "Mac OS X Utilities" application window.
Use the MacOS X Utilities window to run Disk Utility.
if it needs repairing does that mean the computer will be wiped
No, it will not wipe the computer. It will check for bad blocks and repair missing entries in the file system tables. But it is a standard precaution only to do this, if you have a current backup, and check, if the backup drive is working and TimeMachine can read it.
I think there may be an issue with your drive or the file system on it, if the download cannot be opened.