Wallpaper and Wallcoverings - Extensive mildew under wallpaper

We bought a house that had been almost exclusively decorated using wallpaper. After several years of wear and tear, we're ready to remove the wallpaper and paint instead. I started by removing the paper in a small downstairs bathroom, only to find significant sections of the wall covered with mold or mildew. Not sure which it is, most is black or brown, some powdery, and a few small spots are yellow or purplish-pink. (We had, at one time, had a roof leak near that room, long since repaired.) The wall underneath the paper is textured and painted with a semi-gloss. How can I tell if the problem is surface-only and, if so, how best to clean it for repainting? On the other hand, how can I tell if the damage is more significant and, if so, how to repair? My husband and I are both novice do-it-yourselfers.

It should very obvious if the drywall needs to be replaced and that is if it's soft/deteriated. If it is, then cut out the damaged area from center of stud on both sides and replace with the same thickness drywall. screw in place, tape, mud, etc.
If it's not soft but has mildew (matters not what the color is) wash it with bleach, allow time to dry then prime area with a good sealer/bonding primer. Paint to you desired color.

Wepaper: Thank you so much for your reply. Your suggestions were very helpful. Now that I have the wallpaper off and have cleaned the mildew off, there were a few spots around the counter (not large, the largest maybe two inches in diameter) where the paint and the paper on the outside of the drywall actually peeled off during cleaning, so it's stipped down to the "chalky" part of the drywall. Can the drywall be sealed somehow and painted, or will we need to cut those areas out and patch them?

cjcollier, your welcome. As for minor areas, you can fill these in with regular dry-wall compound, allow this to dry then sand smooth. Just be sure to prime prior to painting or wall covering and there is a mildew-resistant primer available. It's a Zinsser product. Good Luck.