Danny's

For only 4 doors 2.5 l should be more than enough ..

Ok. first of all you are thinking at to many layers of paper/lining paper.
lining paper is used for really damaged walls , not on top of existing paper.
if the old paper is strongly bind to the wall and just a few patches here and there , recommend fixing the patches and wallpapering on top of existing one.
you can also just remove the old paper with new paper , no lining paper required.

Needs repainted, it was just refreshed for sale.

Paint dont adhere on mastic. if the joints have mastic , try an undercoat binder...