There is no difference with how low you can between a 18 and 19in wheel. the only difference is the size of the tyres side wall, obviously to keep your speedo reading correctly you have to have a smaller side wall on the 19in.
If the wheels were the same width and offset then its purely down to tyre choice.
Quality 19 tyres at 8.5 are really expensive.
As i said you have the coils so i would go 18in wheels with hubcentric spacers, you might need 15 mil though to fill the arches, and a few people think this is a bit naughty. But i dont know anything about spacers to be honest.
And you normally only get people posting up saying beware of spacers look what has happened.
Also what size tyre would you recommend for the 18" 7.5 et 54?
Cheers
i think but i am not sure, 215's 40 should be ok with 7.5. 225 will definitely fit fine but for the looks you would maybe want a tight fit within the safety limits of course.