I'll take a look at the links in a moment, but unless a single goat will have adequate companions, she will be lonely. Goats are strong herd animals and while they can be integrated into other groups of animals, it will take some thought.
A happy goat is a goat with other similar animals to live with. Sheep are the next closest in terms of care, but a doe producing milk have needs that are higher than in the non-milking season, and are often far different than other livestock.
Most goats who are companions for horses are not producing dairy does.
Most decent quality milking stock start at $300.00 and up.
A good comparison goes like this: A family of four (two adults and two teenagers) can go through a gallon of milk a day.
Said gallon costs say... $3.00.
One month of milk = $90.00.
A quality producing doe can give between 3/4 to 1 1/4 gallons a day (there is a range that matches the needs of a kid as it grows, called the lactation curve) so on average a good doe should be able to pay you back her purchase price and care costs on her very first lactation cycle if you just drink the milk.
Most goats average being in milk 6 to 9 months out of the year. A terrific doe (like several I have) will milk right through for 18 months straight, or until your hands need a break so dry them up for that reason.
If you cook, make soap, cheese, etc... the value just goes up from there.
Whatever goat you choose, as a lot of questions!!!! (Um, a lot of questions as there no question is too silly to ask.)
If you are looking at a youngster ask about the production of the parents. Ask to see them, or pictures of their conformation and udder lines. You don't want a goat whose udder breaks down after just two seasons. It doesn't really "break" down, but if the supporting ligaments and structures are bad (come from bad lines) the udder will be subjected to damage as the supports fail and the udder stretches out with heavy loads of milk.
If you are looking at an adult, ask to see her handled, or put on the stand to see how she behaves. Ask the owner to milk her to see how she stands.
Above all, ask health questions. There are many diseases like CAE, CL, and Johnnes out there that are not always apparent, but will do damage down the road.
I'd be happy to send you some good goat links and info if I haven't already. Sorry, I forget who I've sent what to :)