Thanks everybody for the replies!
About the fan I am planning on installing: yes, it's on a thermostat. But I will install a digital timer in parallel to that and plan on setting the timer to turn the fan on 4 times per day (every 6 hours) for one hour each time. I don't really care about the thermostat and will either set that to the highest temp setting or maybe even completely disconnect it. The main purpose of the fan in my case is to ventilate and remove any possible condensation, not attic temperature control.

Jim, as for sizing the fan: I have done a lot of research on that topic, and I believe that one can make two major mistakes here:
(1) is undersizing the fan with regards to the open intake area. In this case, it becomes pretty much useless, as it will suck the air in mainly through the other ridge vents that are close to the fan, and there will be little to no added benefit in the lower soffit areas.
(2) is oversizing the fan with regards to the open intake area. That would have two negative effects: it would create a negative pressure in the attic, possibly causing air to be pulled out of the living space. That would increase the heating bill unnecessarily. Another negative side effect of this would be that the fan would be running overloaded all the time, and that could drastically shorten its life.

As I said, Broan recommends an open intake area of > 5.3 sq. ft for their 1,600 CFM model. My open intake area sums up to 5.8 sq. ft so that's probably the closest I can get to an optimum sizing. So I'd expect a significant amount of air to be sucked in through the soffit vents, without too much negative pressure build up. Yes, I am an engineer, and I have been up there and measured it out ....
As for the volume, that fan is theoretically capable of exchanging the entire attic air volume every 7.5 minutes.
Am I missing something important with regards to the fan?

IMO, an additional benefit to the fan is that spores in the attic air will be sucked out instead of having the chance of somehow entering the home. But I am really not too concerned about that, because the mold tests have shown that (a) it's not the toxic stuff and (b) it appears that none of it has been coming into the living area at this point.


So I hope to be fine with my solution. Another thing about replacing everything right away: that could back-fire badly IMO. For it to be a long term solution, the source of the moisture needs to be removed. I suspect condensation is causing the moisture, combined with the lack of proper ventilation (blocked vents). But there is no way to be 100% sure. Doing some mold clean up now and fixing the ventilation will show me if the mold continues to spread and/or comes back. Maybe there is another issue in addition that I have not discovered yet? Imagine I spend $10-$15K now for the all or nothing approach and then next spring the mold is back? That would not be good. Fixing what appears to be the source now and then monitor the situation seems like the more feasible approach to me.

But as I said, I am just and engineer and first time home owner, and by no means an experienced home inspector or mold expert. That's why I am here asking my questions. And thanks a lot again to everybody who has replied / will reply. Any additional input is highly appreciated!

- Mike.