Firstly, your old pet is going to be very harassed by the new ones, so it may be better to wait a while or get only one pup first. It is never a good idea to get 2 puppies together, as they are a handful. Secondly, getting a male and a female can once again can be a problem when the female comes on heat. Unless you are prepares to neuter/ spray the dogs. Thirdly, the choice of breed will depend on your living conditions and whether or not the breed you like is suitable to live in those conditions and if you can take good care of its individual breed needs. Lastly, the cost will depend on the breed and whether its a show specimen or a pet quality, however one should not compromise on the quality as it is a 10 + yr commitment even if you have to pay a little extra.
See buying a new pup section for more tips etc.