My computer's harddrive was recently reimaged. How do I get the songs that were in my I-Tunes before loaded back into my newly downloaded I-Tunes?

I have an older IiPod Nano, model MC027LL. I'm using 2.6 GB of space, but I have 4.7 GB of space remaining. I've had iTunes for several years now. I recently had to have my hard drive re-imaged. When I got it back, I downloaded iTunes again, signed into my account, and when I clicked on "Purchases" I could see my songs. But, when I tried to download them into my new iTunes Library, it said my computer was not authorized. So, I went to my Account Settings, and removed all my previous authorizations. Then when I click on "Set Up an Allowance" iTunes shuts down. Additionally, when I connect my iPod up to my computer, a message comes on within the iTunes program which says, "An iPod has been detected, but it could not be identified properly. Please disconnect and reconnect the iPod, then try again. If the problem persists, uninstall iTunes, then install iTunes again." Is it possible that my iPod is so old that it is causing these problems? If so, if I get a new iPod, how do I ensure that I can still get my iTunes installed once again onto my new iPod.?