1) How old should a dog be when starting Schutzhund training?
You can start foundation training and imprinting as soon as you get your puppy. For people who decide to start after already having a dog, starting between puppy-hood and two years old would be pretty ideal. However, many people have started their dogs older than that and they took to it very well. A dog in their senior years may have issues with the physical demands of Schutzhund, so better to start earlier.
2) How long does this training take, typically?
That of course, depends on so many things! How often you train, what resources are available, the climate you live in (no tracking through the winter here, and indoor training limits what we can do). But in general VERY general, if you start from the time you get your puppy, you could be ready for a BH at the minimum allowed age of 15 months, but taking 2 or 3 years to do a BH is okay too. Then you can go on to Shutzhund I, II, and III. Most people that I know will do one Schutzhund level a year, but that is not a hard and fast rule.
3) I know I'll get a lot of answers for this one, but how much does training cost?
Each club is run and organized differently. Club membership can range from around 100$ a year to several thousand a year. Then some clubs have extra weekly training fees too. Equipment is not too expensive, but traveling to training and trial entry fees can really add up.
4) Where can I find the rules for Schutzhund?
I live in Canada, so I got a rule book from the German Shepherd Schutzhund Club of Canada: http://www.gsscc.ca
There are a number of US organizations in the states, I'm hoping others will jump in and direct you to a good site.
5) What makes a dog well suited for Schutzhund?
A dog with a correct working temperament. You want a dog with strong drives, energy, willingness to work with the handler, a desire to please, a dog that takes pleasure in the work and the training. A dog that does not quit when faced with challenge and adversity. No sound sensitivity (there is a gun shot test in the Obedience portion), solid nerve, no fear. If Schutzhund if something you will be pursuing, the best way to ensure that you find a suitable pup is to find breeders who works, trains, and titles their own dogs in the discipline, and aim to produce stable dogs that are easy to live with.
You will find more good info here on finding a breeder and learning more about Schutzhund. Worth the time to go through the articles: