i have 18"x8" rims with 235 tyres all round on my R32 GTSt & i have no problems at all. i tried wider tyres but anything over 235s rub on the guards.
my car is lowered 2.5" all round & i have had the guards lipped @ the back, thats it, the tyres never rub or scrape & the ride height is perfect! i had the car lower but 2.5" is the best compromise between looks & handling.
also depending on the brand or wheels you buy, you may need spacers on the front to clear your brakes, i have spacers on mine but i havnt had any problems with them, ive heard sometimes you can feel the steering shake when you travel over 80ks with spacers but like i said i havnt had this trouble with mine