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