Noob question about buying the base game and Legion

My friend wants me to get into WoW with them and, after playing the starter edition of the game with a few characters, I feel comfortable enough to buy the game and start really playing. But I have a few questions:

• If I apply the code from the base game that gives you a month of gametime and hypothetically the code for Legion, would that give me two months to play or would it not stack?
• Would it be a good idea to buy Legion immediately (for example, tonight), or wait until I have characters that can actually access that content? (This is a weird question because I'm actually really interested in playing a demon hunter and might even main the class)
• If I do buy Legion, is it possible to just get a standard box for it (something like the battle chest for the base game of WoW, which I now own) or do I have to go digital for things like the level 100 character boost?