Mate are you sure about the widths? The width measurement should be taken in-between the outer rim/flange, so only the flat part when you look from top and the wheels are vertical. I say this as I made this mistake recently
I run 235/45/19 & Goodyear F1 Eagle GSD3 275/30/19. Depends on offset but 285 is probably max on rear without problems. If you're not running big power then 275 on the back will be fine.
My car is lowered, I have no probs on the back. The front tyres rub with the front of the arch lining only on full lock.
As for the nuts - It depends on the wheels. I had to use different nuts when I moved from my 18"s to 19"s.
Your car height will probably go up slightly.
Edit - If you know of a local place that puts tyres on for cheap then try http://www.Blackcircles.com
Cheers,
Ro