Before picking out your cat or kitten, you may consider having these essentials available before even starting to look. 

          Veterinarian                                              

Choosing a veterinarian before your pet is good idea so that you can have the new cat examined as soon as possible.

Nutrition

If possible try to find out the type of diet the new cat has been on.   If  you want to change the diet, mix the new food with the old gradually adding more and more of the new.  This is easier on the digestive system and may help any of those little messes that come with an upset stomach. Keep table scraps at a minimum and NEVER feed your pet raw meat, due to the fact that it can lead to internal parasites.

A cozy, warm bed

Whether you purchase a bed in a commercial pet store, or make one of your own, using a box with a nice warm blanket, giving your cat a place of his own leaves him with a sense of security and MIGHT  keep him off yours.

Brushes/combs

Essential, especially for the long haired cat.  Brushing should be done
on a daily basis paying close attention to under the limbs and behind
the ears.  Occasional professional grooming would be a good idea.

Kitty Litter Box

When purchasing a litter box make sure that it is big enough for your cat to
move around in it. If buying for a kitten a small pan or container may need
to be used until he or she is big enough for the litter box.  When choosing
litter you may choose the larger clay litter or the finer clumpable type.
Your finicky feline may even have a preference for one or the other.  You
may want to give both a try.

          alert
          friendly
          responsive
          comfortable with people                                                        

The above are desirable qualities in any pet, whether you are looking for a solitary one or one that is more active.  A kitten or younger cat is usually very rambunctious and looking for stimulation to keep him or her busy.  They would be much happier with another pet to entertain itself with, especially if you are going to be gone for long periods, such as working.

If you are looking for a feline with a more solitary nature, then perhaps an older one is right for you. Generally speaking, cats that have reached the age of approximately 4-5 years usually have settled down a bit.  The older homeless cats have a much harder time finding homes so if you interested in this type of personality be sure to check with your local humane society or animal shelter.

            clear, bright eyes
            absence of nasal discharge
            healthy teeth and gums
            no discharge present in ears (if discharge is present it may be a              sign of mites, bacterial, or yeast infection)
            a shiny coat with no evidence of bald patches, dandruff or flea dirt
            well-formed stools

If it is at all possible your best move would be to take the cat to your veterinarian for a general examination before making the final decision. Even if the animal looks healthy it may have worms or other problems that you need to be aware of.  For the older cats a FIP and Feline Leukemia test would be an excellent start. Normally you are given a trial period with your pet before the transfer is final, but you might want to check with the owner before taking him/her home.