The only dual layer disks the vast majority of us recommend are Verbatim, preferably the Singapore made Verbatim, since the ones made in India seem to be less consistent.
Buying any other types of dual layer disk is just a crapshoot. You might get lucky, but probably not.
The most recommended single layer disks are Verbatim and 8x Taiyo Yuden.
And to back up your dvd collection, you can either use free tools or commercial ones depending on your needs. I suggest looking through this threadfor free tools. If you just need full ISO’s to burn to dual layer Verbatim, you can use DVDDecrypter for most dvds, and DVDFab HD Decrypter for those dvds that have newer protection schemes. Use ImgBurn for burning dual layer disks.
I use AnyDVD to break encryption, DVDShrink on the rare occasions I use a transcoder and DVDRebuilder Pro for most backups that need compression. And I rarely burn dvd video with anything other than ImgBurn anymore.