The import script will maintain your forum structures, thread structures and user information, but for more accurate information on exactly how it will function you would be better off asking someone from vBulletin.com...
The "Premium Membership" feature is actually included within vBulletin 3.x (Any vB version above 3.0.0) - Try out the vBulletin Demo board on vBulletin.com to see how this works
As for vBGarage, I am currently unaware if a version of vBGarage exists for the latest version of vBulletin, but I'm sure as it is a popular mod, it will be released...
As for modding of vBulletin, it depends entirely on how skilled you are; Most modifications are simplistic to install, some don't even require you to login to alter any files, but some complex modifications may require file edits... That being said, it's fairly easy as long as you follow the instructions given by the coder on how to install the modification...
Chris