Don't buy a cat... seriously... go and adopt one from a shelter instead. They put cats to sleep there just because there are too many, and it's sad.
This time of year they will probably have kittens, too, if you happen to want a kitten. (Pluses: You get to know them from the beginning. Minuses: They're little hooligans at that age.) They are going to charge you for adoption fees, but that will be no more than what you would pay at a pet store, and you generally get their first set of shots along with it. If it's an adult you're probably going to get one that's already neutered, too, which saves you money.
Two cats is a good idea though; and if you get them as kittens they will probably either tolerate or like one another as adults. They keep each other entertained, so when you're away they don't get bored and destroy things. (But get them a good scratch post or two anyway, and show it to them whenever they go to scratch something, or they'll want to scratch your furniture.)
