Would any of you recommend installing a whole-house fan? Keep in mind
that I'm in New Hampshire and July & August are probably the months that
get really hot.


A whole house fan would probably be very useful in your area, where
there are many times the temp drops down nicely overnight. This would
quickly cool the house down close to outside temps and it could stay
reasonably cool till at least mid day.


And if yes to the whole house fan, is a powered vent required on the
roof? I was under the impression that eaves+ridge vent would be enough.


All that is needed is enough exit area for the fan rating. That is
specd by the manufacurer.


Again, the fan itself is not too expensive, but the installation & labor
makes me think twice. As an example, the fans can be $200-$400, but
the installation estimates are between $1000 (without powered vent for
the roof) & $1800 (with powered vent).


Sounds high to me. The whole house fan is more involved, because it
involves cutting/framing an opening in the ceiling. But $800 for a
powered attic fan is way high, unless the install is complicated for
some reason. You can get a fan for $100 that will install either in
a gable or in a new hole cut in the roof. I'd say it should cost more
like $400 installed.