Bullseye is a high grip primer which adheres to almost all surfaces without sanding and also acts as a stain block. The Plus version is a newer spec product which does even more, and is also alkali resistant.
Gardz is a clear penetrative sealant which helps on porous, damaged, problem surfaces and dries to a hard film that accepts most paints well. It is also designed to seal in wallpaper paste residue whereas the Bullseye won't, meaning any residue would reactivate and bobble as it gets wet.
If the wall is completely cleaned of paste then either product can be used, although Gardz is the cheaper option. It is pretty thin though, so you will need to bear that in mind if you are not neat with a brush/roller.
Rag rolling can normally be painted over quite easily but, if the texture is thick, it may still show through subsequent coats unless it is sanded.
The other option would be to line the walls before painting, as has been suggested.