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If you watch the 1st Doctor serial called "The Reign of Terror"(1964), the doctor says it is impossible to change history. In this episode Barbera tries to prevent Napoleon from becoming ruler of France No matter what you do, time will right itself. The Daleks had developed time travel in the 1st Doctor serial called "The Chase"(1965). Try torrenting classic Doctor Who. You will really enjoy it. Also, the time lords have their own time line that can not be interfered.

To answer the second question first, the Daleks originally developed time travel specifically to pursue the Doctor. They first used it in the 1965 serial The Chase, starring William Hartnell. It was the third story to feature the Daleks, and they had developed a time machine in order to pursue and exterminate the Doctor and his companions. From then on the Daleks' time travel abilities became more advanced. They followed the Doctor again in The Daleks' Master Plan, travelled between 1866 and 1966 in The Evil Of The Daleks, travelled back in time to invade Earth in Day Of The Daleks, used a time corridor to link between Earth in 1984 and a spaceship in the future in Resurrection Of The Daleks, and travelled to 1963 in Remembrance Of The Daleks. In that story the Doctor describes their time travel technology as 'crude and nasty', nowhere near as good as Time Lord capabilities.

As for travelling back in time, the very first outline for Doctor Who back in 1963 specifically said that such use of time travel is not permitted because there is no dramatic hook if the Doctor can simply go back and fix things. In the early episodes it was considered inmpossible to mess with history. In later serials it was portrayed as highly dangerous to mess with the timelines. In Father's Day, for example, Rose goes back to 1987 and saves her father's life, and all sorts of things go wrong which nearly result in the destruction of Earth until the timeline is put right again.