What kind of grain should i buy for my horses?

I have found a horse i want to get and it's a percheron/belgian and 15hh and 2 y/o. What does 15hh mean? I would really be thankful if someone told me. It's a girl and it's called a filly. What type of grain should i buy her and how much of it should i feed her? I have already deciced i would get a round bale and let her snack anytime she wanted to make sure i didn't feed to little hay. What are good types of hay to feed that come in round bales?