I'll be surprised if the whole story is about getting the Falcon, though I'm sure that'll be in there somewhere. Given the cost savings involved in simply reusing the sets they already built for TFA, I'd say there's a slightly better chance that it'll happen fairly early on. Who knows, maybe that's how Han & Lando first meet: across the sabbac table?
Not sure about the timing, but it seems Lando already lost the Falcon by the time he shows up in SWR, which is roughly 4 or 5 years before. That said, Alden Ehrenreich is in his mid-to-late 20's so if he's playing Han at around the same age that would place it right around that time. Who knows, maybe the movie happens right before Lando goes to Lothal? Another possibility is that we'll see the Battle of Taanab?