Is it absolutely necessary to take a guided tour of the Great wall?

Of course you can go by yourself. You can take the bus to Miyun County at Beijing Dongzhimen Long Distance Bus Station, and take the local tourist bus to Jinshanling in Miyun County.
The scenic spot will close at 17:00 in the afternoon, so whether you can see the sunset depends on when you plan to go to the Jinshanlin. If you go there in the summer, you may not see the sunset, but you may can see it in winter.