Need help determining Horde character questing path
Back in MoP I had a human Death Knight, which I highly enjoyed playing. Then I've preordered WoD and boosted a human mage. The reason I did this is because I wanted to learn all the lore that is available through questing/dungeons/raids and I've skipped a lot of reading while RaFing with my friend. So I needed a character with no quests completed and the ability to solo old dungeons/raids since it'd be difficult to find a group of people willing to wait for me exploring the lore (which I'll still have to find for the content I can't solo currently). But I couldn't start because I was overwhelmed with the amount of things I'd have to do.
Now I'm reading Warcraft lore books and planning to complete them till February. Then I'll buy a laptop and start playing my mage until I complete all the quests. And this is where the problem comes up.
I also want to experience the horde side of Warcraft universe and will buy a Legion for a boost, so I could go through the content without putting too much time in actual killing mobs while questing (probably still going to take a lot of time even though I'll one shot most of mobs).
I don't want to repeat quests on my horde character, like those who have the same/different quest giver but the same objective. So how can I find out which zones I should go to and which quests/questchains I should do? E.g. maybe I'll have to repeat some alliance/horde quests just to get a reputation and unlock some horde specific quests. Also are there gungeons/raids that provide different dialogs for depending on your faction?
Also does anyone knoe how many hours it will take me? And do you think they are going to eliminate some lore important content, since I heard they will make some changes to our beloved Azeroth? Hopefully I didn't confuse you.