Use an oil-based (alkalyd) primer that is compatable with waterbased paints.
The primer will penetrate the water based paint as well as cause any loose edges to curl slightly. Repair these areas, prime again, skim, prime then paint.

Guardz works fine - no specific need to use an oil based primer if fumes are a concern. Sanding the walls is a good idea, then guardz, then a little more sanding if necessary, skim, skim, skim again if you want it perfect, prime and finish with premium quality materials. Zinsser's Kitchen and Bath paint seems to work well in high humidy environments.

Or

If the walls are not painted with semi-gloss or have torn drywall paper showing, simply sand the walls and start skimming. Priming is usually not necessary over flat or low sheen paints. Do use a high quality primer over skim coated walls prior to painting with 2 top coats.

GARDZ will serve all your needs. GARDZ prior to the skim coat if there is any damaged wall area. skim coat, then GARDZ again. GARDZ is a primer and can be painted over with your topcoat.

there is seldom a need for oil primer or paint inside the home.

Zinsser Perma White as Randy suggests is a very good choice.

Michael