Let me say, straight off the bat that I am not talking about a puppy mill or anything of that nature. What I am referring to is having a few dogs which you either use for breeding or ones which also work on your farm and you sell off the pups.
Firstly let's look at what breeds would suit which type of farm. What do the people in your area require? Have a look in the paper or on the internet to determine the need and the scarcity.
For example, in my area of Brazil, many people want guard dogs. So people breed and sell Rottweilers, pit bulls, German Shepherds, Dobermans, and one called a Fila Brasilero which is a Brazilian Mastiff.
This is the traditional market of dogs, however there is an emerging middle class here and people from the city want apartment dogs. These include the teacup poodles, miniature pinchers or (min pins), and Yorkshire terriers. Apartment dogs are big business where I live.
If you are merely having them as breeding stock, kennels are the best option. No one wants 10 puppies running around their home. Purpose built kennels will show you are a business and not just breeding a few dogs to cover this month's bills.
Where I live, puppies are sold at 2 months which, in my opinion is too early. I believe 3 months is the best option. Ensure your puppy has all its injections and that the worming is up to date. Start advertising your puppies soon after birth so you can guarantee a sale. If the buyers have to wait 3 months, they will know they are getting a healthier puppy because of it.