You shouldn.t need to buy a new OS as long as you have the product key code. You can use any installation CD of the same version as is currently installed.
Backup up important data now and consider backing up the drivers into the bargain. Dell usually has them in a drivers folder on the C drive.
Don't trust a drive that shows any sign of failure. The restore partition could be imaged to DVD or CDs or to an external drive using BiNG. In fact all the partition s could be imaged in this way if you have enough space. The images can then be used to restore the system onto a new replacement drive.
Personally I would get a new drive in a usb enclosure and clone the current onto the new drive. Then swap the drives from enclosure to laptop and use the original drive as a backup device - for as long as it continues to function of course.