You should ask if the heifer has had a brucellosis test (or vaccine,) a TB test, and especially, a Johne's test. If she hasn't had these you should get a vet to do them all before you buy her. Have them test both blood and stool for Johne's. We have Jerseys and Dexters, and we got burned a couple of times buying Jerseys who turned out to have Johne's. One of the cows was fat and healthy at the dairy, but the stress of moving her brought on the Johne's. She pooped all over our barn and contaminated everything while we waited for the test results to come back. Johne's is a disease that can lie dormant for years, then surface when the animal is stressed. It causes malabsorption of nutrients and diarrhea, and is spread in the feces. The organism (mycobacterium tuburculosis I think) can live in your soil for a year or more. The cow literally starves to death no matter how much you feed it, at the same time contaminating your property. It's a widespread problem, especially on dairies, and in many areas it's just starting to be recognized for what it is, so a lot of dairies don't have testing and eradication programs in place yet. First calf heifers who have been exposed to Johne's often look fine till they freshen, then quickly go downhill.
As an interesting side note, we've read of several cases where cows or bulls have had what appeared to be fatal cases of Johne's, were taken off all feed except good pasture and all the kelp meal they wanted, and recovered completely. A couple of the animals were even autopsied at death and showed no signs of the disease! We now feed our cows nothing but grass or hay, free choice kelp, and free choice loose white salt. The milk cow also gets some alfalfa hay or cubes and makes as much milk as she ever did on grain. She never was a huge producer, but makes much more than we can use - around a gallon and a half a day.
Oh, one thing you should check the cow for is teat size. It takes forever to milk out a cow who has tiny teats. Trust me, if you ever buy a milk cow with small teats who you plan to hand milk you will regret it. Been there, done that too :no: