-What can they eat?
As calves they get a mixed and fortified grain and corn mix that you can buy at a feed store as well as some hay. (Timothy or Alfalfa) as they get older I think they just eat grass and hay.

-Are they rideable?
Yes, but not all will train and not all are comfortable. Most riding cows are Texas Longhorn Steers. Dairy are harder to train. You might want to check out these websites:
#1
#2

-Are they good pets?
Yes, quite lovable and friendly. They'll moo when they see you coming even if you don't have feed.

Other info
-The average show calf starts at about $1000. You can get crosses as just a pet from rescues though. Try looking on Petfinder.com . I have always loved cows and now I have one!