If you do decide to wallpaper, it really isn't anything to be afraid of, honest! Lining paper, being plain, means you haven't got to line a drop to the next horizontally (as with patterned wallpaper) just vertically. Still use a plum line, a large screw tied to a piece of string will do, use the corrct water to paste ratio, mix it well so no bubbles, paste right up to the edge of the paper, smoothing brush up to the edges when it is on the wall, lightly score top & bottom of excess paper with your fingernail and trim (scissors are better than a knife). Oh and don't forget to size the walls first with watered down paste mix. It sounds difficult but it will probably be quicker and easier than 15 coats of undercoat!