The MY2018 M550 is now in BMWNA's ordering system for the US market, and I ordered one for 04/14/17 production.
I have no idea how much it costs beyond the already-released base MSRP; all the option and package prices are listed as "Blocked". The dealer expects full option sheets, pricing, and lease money factor/residuals to be out within the next couple of weeks.
There are no Priority 1 options, additional wheel choices, etc., available, and it's unclear what will be available since it is already a M Performance vehicle. If you are ordering a M550, the packages/options can be modified until the vehicle enters production.