Soffit vents? If not, your ridge vents may be totally useless.

If it's "really hot" up there, I'd look into fan-assisted,
thermostatically-controlled ventilation. I did recently, as did a
friend, and it works great in terms of avoiding roasting shingles and
heating house from above. In your case, you may want the fan blowing in
with exhaust along ridges.

As to where to put unit, I'd ask the installer- you may need all new
ductwork. Furnace may enter discussion, too.

Moisture problem definitely should be addressed, with vapor barrier,
ventilation, exterior swamp drainage, whatever works. DAGS on this and
await a deluge of hits.

HTH,
J