You wouldn't have to get a pure bred, it might help in resale or the selling of offspring though.
What you need to look for is a goat that handles well, fairly tame with a nice sized bag -not meaning huge, but rather 1/2 a basket ball, snug under the belly not hanging down too far.
Tits nice sized
(sorry) (you have to be able to get at least 3 fingers on it-if first time milker this would be acceptable they will develope- 4 would be better), but not so long you wonder where to put your hand. you should see a difference in the bag and the tit.
A older goat(4-6 yrs) might be best as she has experience and would be more patient while you learn to milk. Ask the owner to show you how and let you milk her before you buy.
As for cost- you can get a goat producing milk for $10.00, but I would be leary of how well she will stand. I would say $50 To $100 for a GOOD milk goat- one that wont make you wonder what you are doing freezing you hands off for spilled milk.
Any goat that has given birth can be milked it is just a question of how much milk you want and how much of a struggle you want to get it
.
You should have a milk stand, it really makes it easier, I have however milked one-who very much opposed- tied to the fence in the pen. If you are handy you can build one fairly easily.
Talk to the owners of the goat you choose , most people will be glad to guide you in getting started.
Hope these ramblings help you. Remember a tame goat!