Bob,

You will get many useful comments from the regulars on this
group. You will also get volumes of information from a Google
search. Two or three hours on the computer reading the
advise on some of the Google hits will be time very well spent.

1) Two layers of shingles is common. But, I'd recommend
tearing off 30 year-old shingles. The new roof will look better;
you'll get a chance to examine sheathing; the roof will weigh
less; etc. Just my opinions.

2) Costs will vary tremendously depending upon circumstances-
shingle selection, tear-off, sheathing replacement, etc. Roofing
cost estimators are available on the Internet. Also, 10 contractors
coming out for estimates will give you an extremely good idea
of what you will have to pay. (Do it yourself and save a bundle!)

3) Brand of shingles - Consumer Reports did an article on this
in the past year or so.

4) Dealing with the contractors - Consumer Reports, homeowner
DIY magazines and such, etc. Get the first 2 or 3 contractors
out for quotes & discussion and you will start to discover the
sort of questions that you need to resolve. Talk with neighbors.

Comments beyond what you have asked: Written warranties are
important. Company history is important - don't buy a roof from
a company less than 10 years old unless you are a big gambler.
Be home the day the roof is installed and let the contractor know
before hand that you (and/or a knowledgable friend) intend to walk
the roof after the tear-off and again after job completion. Don't pay
in full on the day of competion, if at all possible. Walk the yard
after completion and refuse to pay if a quick walk around the yard
turns up even 1 remaining nail. If you spot 1 nail, then there are
probably 50-100 you haven't spotted. The same goes for the driveway
and the street. Ask for reference from roofers, but insist upon
getting the names of future customers and not past customers.

What roofer is going to give you the names of unhappy past
customers? But if you get the names of 5 near-future customers,
then you should be getting a reasonable representation of
customer satisfaction. (You should be able to spot the asshole
customers easily and discount them). I'd want to climb the ladder
and examine somebody's roof just after it has been completed
by a contractor that I am considering. Are the shingle lines
reasonably straight? Are nails exposed when they shouldn't be?
Are ALL requisite exposed nails caulked? Are there any spongy
areas as you walk over the roof? How short are the pieces of
shingle at the edges of the roof - a four inch wide piece of end
shingle will look ok only for a few years until it blows off in a
storm. Is there damage to the edges of the shingles from workers
carelessly working on a hot roof? The list of endless.

Once again, talking with intelligent neighbors is very important.
They've gone through this already. They know particular problems
in your area. They are likely to have checked out and/or used
the contractors that you will consider.

Good luck,
Gideon