I don't know of anyone with 18's but from my playing with suspension and wheels, Id say 18's would fit with rolled guards and about a 7in width 7.5 might fit but I wouldn't risk it offsets I wouldn't know but id be more concerned with inner guard rubbing than outer. My Lx's standard wheels were rubbing on hard cornering at the front inner guards after lowering with d2's. My Attesa's wheels are 17x7 and they had ware marks on the inner guard when I bought them, that was with 45 profile tyres and lowered by kings springs and standard struts. Now it has d2's and 40 profile tyres and theres no rubbing at all. d2's are set to around the same height as the kings.
When your shopping for wheels you have to take into account, amount your going to lower your car, stiffness of springs/shocks, swaybar/strutbar (how much it tightens up the car and prevents body roll) even chamber can effectively impact wether it rubs or not. As well as these you have to look at what you have already mentioned diameter, width, offest not to mention tyre profile. Driving youdo will affect it to, if you drive normaly you can proberly get away with big wheels that would rub if you realy pushed it around corners.
The fronts are what you'll have to work around the back wheel arches have heeps of area I doubt you would get any rubbing around there unless you totally dump the suspension.
Also iv herd of some Americans putting 19's on a Altima (bluebird). You might be able to remove plastic bits in the front of the car to prevent a bit of rubbing I don't know.
Anyway good luck, also check out
www.altimas.net you might find the info you need there.