We adopted two kittens (one 8 weeks, with a severe upper respiratory infection, that required medication, and one four month), three weeks apart. We both work full time, and we were able to cope, although it's a bit hectic keeping up with two kittens! Much havoc was wrought, a lamp broken, etc. They really don't demand that you be there all day long, but do spend as much time as possible with them. Cosmo, my avatar pic, follows me around like a dog.
An adult cat is a lot less work than a kitten, but kittens are a blast. They love to play, and by spending time with them you can seriously bond, moreso than with most older cats. The kitten moments are priceless, IMO.
Depending on the shelter, the cost of spaying/neutering may be included in the adoption fee. Our was, as well as the first vet visit. City shelters tend to be less expensive than private shelters, at least in our town. The local adoption agency wants like $80, plus vets fees. The city shelter was $35, and that included spay/neuter and the first vet visit.
Be sure to spend some time at the shelter with prospective adoptees. Cats tend to choose their owners, and you can often tell very quickly which cat is for you. Cosmo and I bonded almost instantly.
Edit:
Oh yes, and lots of toys. Also a scratching post or mat, to help save your furniture. Don't bother with catnip until at least six months, kittens don't respond to it. A squirt bottle or gun is useful for training purposes, like keeping him/her off the table, counters, etc.
