Pre patch and pre ordering Shadow Lands

I apologise if this is not the correct subreddit to post in but i thought id try my luck. I'm looking to get back into WoW but i havent played it for years on end now. Currently as i dont own BFA i am unable to pre purchase Shadow Lands with out purchasing the previous expansion. My question being, when pre patch hits some time late this month, will i be able to purchase shadow lands and play through BFA content and level before it hits? Or will i need to wait until Shadowlands releases and then purchase it?