Holy cow.... don't spend 10K replacing insulation and your roof just yet!!

Get us a picture or two when you can. The situation you describe is pretty common in my area and often requires little or no action at all. Even according to the 'mold police'

There's little or no air exchange between your attic and your house so the threat to the occupants is minimal.

The fan is a good idea but beware that often times they are on thermostats which don't come on for about 6 months out of the year. Which happen to be the six months when most of the damage occurs. Insulation blocking soffit vents and bath fans not routed outside are cause most of the time. In 02 it was still common to just point the fans near a roof vent and hope for the best. Lately, I've been seeing a lot more that vent to and through a gable or to one of the newer roof vents that gets the air completely outside.

IMO until the moisture is so bad it has damaged the roof decking there is no need for replacement.