What is the best type of mattress to get for long-term health and
comfort? I have been told that the best is a firm mattress (for
healthy) with a padding on top (for comfort).

I could do a 10 minute Google search and get a few people's random
opinions. Mostly, I am interested in  more in-depth, reliable, and
preferably scientific explanations. It would also be nice to know
which brands provide the healthier mattresses. It would be great 
if I could get an answer within the next couple days.

I am interested in this topic as well.  Perhaps you should simplify
your question to get it answered.  "Comfort" is such a subjective term
when it comes to mattresses.  Firm is comfortable to some while others
don't like a firm bed.  Also, "long-term health" is a vague phrase as
well.  I believe that a doctor would recommend different levels of
firmness depending on various variables, such as a bad lower back or a
bad upper back (not 100% sure on that, but just a guess).