if you can wait, I'd wait. it's always easier to get a better deal with on-lot trucks vs. ordering. And being a "first year" production for 2019, waiting a year or so may not be such a bad idea. sure 2019 is mostly a major refresh of the 2018 but there's still a lot of changes with the new 2019 so let the potential bugs work their way out of the first production run.
not sure if there's a problem with the 3500 production line. although, I'm not sure if that was the series that was moved or planned to move from mexico to Michigan.