How do I deciding whether to install an attic venting system?
I think an attic ventilation system will definitely help with your problems. Fortunately, you can also sort of step up with ventilation as you go, adding powered fans if the vents you put in at first dont work as well as you'd expect.

In the winter, having a properly vented attic will help prevent ice dams in gutters, and can help with heat problems in the summer too.  In your attic (above the insulation) should ideally be close to the same temperature as the outside.  It should be cold in the winter, and hot in the summer.

For the ventilation, you should have some soffit vents on the bottom and a vent on the side (near the top) or on the top (like a ridge-line vent).  That will allow the air to move freely.

If the air doesn't move as well as you'd like, you can wire in an attic fan to help push the air along.