If you get a bred cow, then there are 2 ways to go. Is she bred but still milking? You are getting a better deal that way, getting some milk and yet she is bred back so you will get a break from the milking when she is dry and then when she first comes fresh, you can start her with "calves" if that is your choice, or milk her only or a combination. The only real plus to getting her as bred, is you KNOW that she is/was/can be bred. Sometimes a cow will just not breed back, no matter what. You don't want to get one in milk, unless she is just fresh, that is not bred because there is always that slim chance that she may not breed back. I had one that did not breed back after her 3rd calf. Not AI, not with a bull, nothing. Vet couldn't find anything wrong. It happens.
The dairy having an occasional cow for sale may be a very good route if you follow a few precautions. Usually a dairy will sell a cow, not for cull, but to a prospective private sale, because she does not make enough milk for the farmer to realize a profit. With the shape of the dairy industry and how little they are getting for their milk right now, ALL FARMERS are finding it very tough to even break even on costs. A low producing cow just won't cut it. It would be an ideal cow for a "homestead". Not too much milk to overwhelm you.
Now there are things to watch out for. Does she have 4 good clean PRODUCING quarters? I am not the slightest bit afraid of a 3-teat cow. She could have had a case of mastitis, it cleared up but the quarter did not recover. You can "kill" a quarter to stop it from producing. It is done sometimes when you can't get the mastitis cleared up in it. Sometimes a heifer will just come in with only 3 operational teats. If a calf is sucked by its herdmates, it can ruin the quarter and sometimes even the udder. I have one now that was sucked as a calf, and when I got her and she freshened she only had milk in 3. So be it. She produces very good out of 3. You leave the other quarter alone.
The quarter could have been injured and they couldn't get it to come back into production. Had one that was butted very hard by another cow. She produced bloody milk for over a month. It never cleared up and I finally just quit milking it. Eventually it dried up on it's own, but I would have killed it if not. Not her fault, no other problems. Sometimes when a cow gets up, which they do back end first, they have been known to accidentally catch a teat between their toes, and they have gotten mashed, or cut wide open. Often there is no saving them if damaged enough. So you wind up with a 3 teat cow. There are lots of reasons. A 4 quarter cow is the preferred route.... but not 100% necessary.
So back to the farm selling an occasional cow. I would talk to them. see what they have to offer. IDEALLY: it would be a younger cow, low producing so she isn't making enough to "pay her way" on the dairy. 4 Good clear open milking quarters. If she has been milking for over 120 days/4 months, I would want her bred back. If milking over 5-6 months, she would HAVE TO BE CONFIRMED PREGNANT. Doesn't matter what she is bred to at that point, many farmers are breeding their lower producing cows to beef bulls as the calves are worth a little more to people who want to raise a few bottle calves. But bred/ confirmed preg is very important so that you are not getting a breeding problem cow. Now if you have a beef bull, then she could come and just get bred by him. Realizing that there is a chance she may have been a problem breeder. Some farmers do not bother to breed back their low milking cows, so then it would be a choice. Your bull would most likely breed her on her first heat. Or you could AI her.
For a less than IDEAL cow, she could be an older cow that just isn't making enough. The farmer may have alot of heifers coming fresh and they will often cull an older cow to make way for "younger, new genetics" as the heifers should be better than the dams, if you are breeding to improve your herd. So there are many times now on farms that they are simply culling older cows to make room for younger cows. They will have udders that may not be as tight, maybe a little sagging as the ligament support loses its strength. As long as you can sit on a stool or 5 gal bucket and get a bucket under her udder and comfortably reach the teats to milk it's okay. You don't want the teats pointing out to the four corners but they may not hang straight down.
You do not want little tiny teats that you can only get your thumb and forefinger around. You need a teat that you can comfortably fit at least your thumb and 3 fingers around. The pinky is not essential but I like a cow that I can get most all my hand on. I have one that has short tiny teats and I cannot milk her by hand. Calves do okay on her but a thumb and forefinger milking is not only tiresome, but a PITA. Also you do not want huge fat teats that you or a calf will have trouble getting ahold of when she first comes fresh. The natural swelling of the udder when first fresh will make it the opposite of tiny teats, but just as difficult to milk.
You want and HAVE to have a cow that will allow you to hand milk her. Some cows on dairies will not be accustomed to that. They only know a machine milking. They have to be accustomed enough to a person working alongside them. YOU DO NOT want a nervous acting cow, or a kicky cow. If she wants to take a swipe at you, LEAVE HER THERE.
Any cow over 10 years old will have a limited life left as a milking cow. I have 2 that are 12 and 14. But getting a cow from someone else I would look for something that has 2 or 3 lactations behind her unless she is a first calf heifer that just doesn't make alot of milk. First calf heifers can be a little tricky as they sometimes don't have the "settled down" thing yet. So yes, a little older cow might be better.
Ask the farmer if he is on DHIA or Dairy One. These are companies that come once a month or so to test the cows. Every cow that goes through the milking parlour will have a milk sample taken from her. It shows the pounds of milk and her butterfat and protein and SOMATIC CELL COUNT as well as other info on that cow. Cell count indicates her udder health and if the cell count is high then she is a candidate for mastitis. A flare up one month is no big deal; several high counts indicates she is a high cell count cow and one to stay away from. She will be a chronic problem down the road. Tell him that you don't know alot about it but you do know it provides info on the cow. If they are decent people, they will show you her records, and explain them to you.