First, I'd check any bathroom fan vents and see where they go. It's
not unusual to find bathroom fan ducts that terminate in the attic
instead of going outside. Having mold grow in an attic like this is
somewhat unusual without a good source of moisture.

The most critical thing is to get soffit vents put in. The attic
already has at least some outlet vents, but no inlet vents. For more
outflow, instead of cutting any more holes in the roof for localized
vents, I'd go with the ridge vent and just leave what's there in
place. It's just as easy and a lot more effective to put in a ridge
vent.

A power vent is only effective when it comes on at higher temps, while
your moisture problem can be present at lower temps too. And if you
look at the minimum amount of inlet area required to support a power
fan, it's considerable, even for a relatively small one. Without
enough inlet area, you will wind up creating a low enough air pressure
so that cool air is sucked from the home interior via gaps at outlets,
etc. into the attic, wasting energy. So, I'd go with soffit vents and
a ridge vent, forget the power fan.