Your little critters health is a very important factor when thinking about buying. Make sure to do research on the species you're thinking to purchase because some of these little buggers can be a pain in your wallet when it comes to taking them to the vet. Diet is also a big factor, make sure you know how much of what you future mammal requires.
Small mammals can be very bitty, so make sure you take that into perspective when looking to buy. You will need to spend time training your pet right and wrong behavior. At times it will seem worthless but it is a must, it also helps if you start training early in life. Quality time training will also help you and your pet bond and form a good relationship.
Your small mammal will need proper shelter. They need to be keep in a quite restricted area. If you own other pets such as dogs your mammals cage will need to be kept safely away from the other pets. They need places to get away and hide. Small mammals can be very timid and cautious.