Baal is in the expansion set. If you don't install it you cannot play the final stage (Baal). Hope i helped.
You just keep playing the last stage.
P.S. Don't uninstall Diablo 2. Keep it and install the expansion set otherwise you will lose everything.
No you will not have to restart, but you will have to change your character to an expansion character.
• 1152 x 600
• 1280 x 960
• 1280 x 1024
• 1600 x 1200
• 1920 x 1200