We had a Lab that had her food out all day and grazed, never had a weight problem. The biggest problems we had with her (and I think it is a Lab thing, as everyone I know who's had Labs says the same thing):
1) They will chew on everything, esp as puppies. More so bc they are Labs and feel the need to mouth everything by instinct. You would be wise to have LOTS of things for new pup to chew on, and even so you'll probably have a few shoes destroyed.
2) Labs need an extraordinary amount of exercise. They are working dogs by nature and 2-3 big walks a day is not too much for them. This was by far our biggest challenge, as we have a small house, we couldn't afford a fence for our backyard, and we were both working 5 days/week at the time.
Sue has a wonderful idea in adopting a rescue dog. My mom adopted an 8yo black Lab that we had to put to sleep at 20yo. (I know!!! Very hard to believe!) She was rescued from the street -- we think she had previously been a service dog, got stolen, and then taken to the pound. She weighed 30lb and had bad heartworms when Mom got her. After a lot of TLC and intensive care, she fattened up to 75lb, lived another 12 years, and was the absolute best animal you could ask for. I get teary just thinking about her.
Labs are wonderful creatures. So smart, so fun-loving, SO good with kids, and so adorable. Whatever you decide, I wish you the best, and we would like to see pictures when you get some! Good luck!