I was exactly where you were at a few years ago. I wanted to raise chickens for eggs and meat. Here are a couple of things I learned along the way about the meat aspect of a dual purpose flock.
First, most"dual purpose" birds sold in hatcheries these days have not truly been bred for size and eating as the breeding skewed to egg laying potential. So, unless you have true heritage birds, bred to reflect their original purpose, you might be disappointed in the size and yield. Even then, a well bred dual purpose bird, will still be smaller and shaped a little differently (less breast meat) that the hybrid broilers that you see in the supermarkets. I was OK with that, but my husband was really disappointed.
Second, an older hen is not a bird you can throw on the BBQ. They are sometimes referred to as "stewing fowl", because they generally are best in slow cooked applications like soups and stews. Many people swear by the flavor of these older hens and find it well worth the effort to cook them down. And, the fat off an older hen is incredible when rendered down. I've attached a link to an article discussing the cooking of traditional fowl, and when to harvest if you are looking for a bird you can fry or grill.
http://www.albc-usa.org/documents/cookingwheritagechicken.pdf
At bottom, if you are expecting a meat bird of the size and nature of what you are used to buying in a supermarket, you will need to raise hybrid broilers (Cornish X). That's the direction we decided to go, although we still process extra cockerels and older hens for food. But, it's still a dream of mine to bred a more self-sustaining dual purpose bird. As Ridgerunner shows, it can be done.