Cost - apart from the obvious cost of buying the puppy and the accessories needed, also bear in mind the ongoing costs. There is the cost of the food, and if you get a big dog with a big appetite this can be significant. Vets bill are another consideration. You can help control the vets bills by getting insurance, but again this is another cost which will needs to be factored in, as well as vaccinations. Also consider what you will do with your new dog when you go on holiday, if you are considering using boarding kennels this can work out quite expensive.
Space - dogs need room to move, and as you would expect a big dog needs more space. Do find out how big your new puppy will grow to, and ensure you can accommodate him. Will he be able to move around your house easily, and how large is your garden Don't forget also, if you are going to let your dog out in to the garden, make sure it is secure, you don't want your dog to disappear off down the road!
Company - dogs are pack animals by instinct and are unhappy when left on their own. Now you can get two puppies or more, so they are company for each other, if you are not going to be at home a good amount of time. However, especially when puppies are young they need people around to spend time with them. Be prepared to spend time each day on exercising, training and grooming your new puppy. It will grow up to regard you as it's pack leader and will expect to take part in family life.
Stability - many dogs, and especially puppies, will suffer stress if there isn't stability in their home. For instance, a new baby, a new partner, and even a new house can all cause stress to your dog. If you are expecting any of these to happen, it is best to delay buying a puppy until you have settled down again.
Mess - unfortunately small puppies can mess up your house. Not only do new puppies have accidents, just like babies do, they may also damage your furniture and furnishings. Ask yourself if you are prepared to live with the stains and smells that inevitably accompany new puppies!