OP, I was going to say "Welcome to the Fest" but it looks like you have been lurking here for a while (11 total posts in just under 6 years)! If you'd asked your question 6 months ago, we could have answered it easily by checking the 2018 M550i Ordering Guide archived on the Fest. Sadly, when the website was refreshed, it looks like all that valuable data disappeared. All I can tell you is that the 2021 Ordering Guide says ZDH includes adaptive suspension (essentially shock which can soften or firm up), Integral Active Steering (rear wheels turn when you are cornering) and Active Roll Stabilization (anti-sway bars which stiffen up to help keep the chassis more level in turns).
All those features may work great on the G30 (they worked terribly on the two F30 3 Series we had with the comparable package), but I think most folks who spec them on the M550 love them. But they also add a lot of complexity, and expensive bits which can break when the car is out of warranty. Since the cars you are looking at are not CPO's (not at BMW dealers), I'd factor that in.
There are plenty of BMW VIN decoders out there. Just Google BMW VIN Decoder.
All US market M550i's come with Multi Contour seats. And the front of the bottom cushion absolutely, positively does tilt up and down. You may not have been manipulating the controls properly, or perhaps the control is broken on the car you are looking at. But the fact that you called them "Executive" seats makes me wonder if you are outside the USA. Perhaps even in Oz?
The US-Spec 530's and 540's have the sport seats as a default, and the Multi Contours as a stand alone upgrade. In 2017 through 2020 you could tick the ZLS Luxury Seating box and add ventilation and massage to the mix. Doing so made the seat surfaces stiffer (due to the inflatable massage bladders sitting directly below the leather), and also limited the degree to which you could snug-in the side bolsters. I had ZLS on my 2014 F10 535 and again on my 2017 G30 540, and switched to the non-massaging, non-ventilated Multi-Contours. I find them much more comfortable, but YMMV.
If I was shopping for a 2017 or 2018, I'd skip the ZDH and the ZLS for reliability and comfort reasons respectively.