Originally posted by Nightshade
Offset I am not too sure on so I will let someone else handle that question.
As for tire size I am assuming you are getting 35 series tires?
215 and 225 is the width of the tire in mm and 35 denotes the tires height (I believe it means 35% of width..not sure though).
I would assume the 215 would work fine with it but remember you aren't going to get the smoothest ride from a tire that thin so expect som harshness in your ride as well as to avoid all potholes you realistically can.
With an 18x7.5 wheel, +42 might rub a little with more than 2" drop, but uncorrected negative camber might compensate.
I have +45, 1.5" drop in rear, uncorrected camber and haven't rubbed at all.
I'd go 225/40 18.
On the second number...35, 40, etc. It's actually a ratio of sidewall to tire width, but it's not a direct correlation of tire width, ie, the 225/40 doesn't have a sidewall that is 40% of 225.
Hope that helps.