Well, it appears, from looking at the episode list on Wikipedia, that many of the '60s serials have some or all of their episodes missing. From the '70s on they seem to be complete. What was the reasoning for BBC losing these episodes?
After going through the available episodes on Netflix and Amazon Prime, they're almost identical in what they offer, but each has a few serials that the other doesn't have. There's definitely plenty to choose from.
There actually used to be more missing, but, some have been recovered. Copies of the episodes take up room, and back then, they had no idea anyone would be interested in watching a repeat of a television show, so many were wiped and the film/tape reused. Pertwee's era is now complete, but it too had missing episodes and some of the episodes were found in black and white. Now the b/w episodes of Pertwee's episodes have all been recolored and it's complete all in color. Sadly, the first two Doctors they have many still missing, some of the episodes have been animated to make complete stories
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?