Hello James,
So long as they have been serviced properly and have the service record I don't think there are any flaws or any worries to look out for. With that I would recommend buying from a Lexus main dealer even if the prices are higher than others.
I can only comment from my own experience - from the years you are thinking about 2016 - 18 the main differences will be the later year (2018) will be the facelift version with revised front grille, tailgate, lights and trim, dashboard switches and larger info screen as standard plus other bits and bobs.
I think it will depend on what you prefer looks wise. The prices seem to be quite high still at the moment - I bought mine a year ago from a main dealer (2018 car) and the prices now for the same spec / year car are substantially higher that what I paid.
Perhaps with the new model out now the prices will gradually recede on the earlier cars - although having been in the new one I was dissapointed and personally didn't find it as nice as the original.
Good luck in your search