I think that first off, you need to answer two simple questions. Do you have the time to devote to a pup. We usually figure about forty minutes a day for a pup, and about thirty for an adult. The, how important is it to you to train your own dog. If the training is a huge deal, then a started dog is not for you.
Money is another factor. Most good quality pups will run about 1/2 to 1/3 of what a trained dog will run.
If you have the money and not the time, look at an older dog.
Started dogs usuallly can fetch to hand, do long singles or short doubles, are collar conditioned, and have been exposed to live birds and guns. If you want a dog that is totally ready to go, you will have a harder task.
If you do decide to go with a trained dog, make sure that you have a list of what you want, and talk to each trainer to see what he has. Make sure you go visit each prospective seller, and get references from people who got the type of dog you want. Make sure that the dog has lots of time in the water, has worked through decoys, and out of a boat.