Do you mean private primary care practice specifically? It is difficult to start up your own private clinic from scratch. If you are looking to join a private practice and eventually separate, going to residency in the area you'd eventually like to work would be helpful from a networking standpoint (don't sign a noncompete!). What med school you go to affects everything to a variable degree (your training, your impression of different fields of medicine, boards prep, regional competitiveness for residencies, etc).