The classic Dr. Whos (Doctors' one through eight) are mostly available on Youtube. There are several episodes of the first few seasons that have been lost forever. The new Doctors' you can get pretty cheaply except for the last season.
By the way, there are over 450 hours to watch... there are many great Dr. Who episodes. There are others that are not worth watching. I would pick and choose rather than watch from beginning to end. The internet will tell you which series are good watching.