Welcome!
A big thing to keep in mind if you end up leveling in retail is that the 1-60 questing zones were revamped as part of the Cataclysm expansion. The only way to experience the story as it was in the original game is to play classic. Now then…
If I want to chronologically experience all of the story…
I’ve played this game for about 10 years now and can say that I’ve paid more attention to the story in the past 3 weeks in classic than I have in my entire retail experience. That being said, I think that if you want to have an authentic, story driven experience, you should level 1-60 in classic, then switch over to retail and level mostly through questing from 58-120.
This is a pretty lengthy process though, and can be bypassed by watching 2-3 hours of lore videos on youtube.
Does classic let me play all of the expansions up to Legion?
Having a subscription to WoW enables your account to play both classic and the retail versions of the game. The retail experience you get with a $15/month sub consists of leveling content from 1-110 (revamped azeroth quests all the way through legion)
You do not use the same characters in classic and retail, think of the two as completely separate games
How does each expansion work within Classic?
There are currently no expansions for Classic WoW, only the base game is available.
At what level am I able to interact and meet up with my friends who are different races in both Classic and BfA?
You can group up at any level with your friends in either game, though the closer in level you are, the more content you can progress through/complete together.