I have low bandwith so cant download much would it be okay to backup drivers and (even if old) install them whenever buying a new gpu, will they update by themselves or will they be stuck on the version i downloaded (nvidia)?
You will be stuck with that version that you downloaded. Sure you can download and keep the installer but it will be outdated once a new version is released.
If you buy a new card it will come with a disc that will have a newer version of the drivers. Then whenever you can get on Nvidia.com and get the latest and greatest.