There is only one thing which could be problematic here - brake discs and calipers size, but this should not be problem in case of Niro.
Different brake discs and calipers are used almost only when you have different engine power/weight of a car and better brakes are necessary to stop the car efficiently (or in case of a sport versions where you need to brake more often and rapidly).
I do not believe that they decided to change brakes in the highest model. There are the same engines in all versions (except PHEV which have more powerful electric motor).
Tires for 16" should be much cheaper than 18" version which have much lower profile and are wider 225/45 R18 vs 205/60 R16.
Take into consideration that 18" are alloy only, when 16" is two parts, plastic cover screwed to the light alloy, at least here in Europe. Plastic painting is very good quality.