Doesn't your car currently have 235s? I think they came that way anyway.
When you upsize wheels, you have to go smaller on the tires. A little difference is usually acceptable, but the BMW doesn't leave much room for error. If you visit this site:
http://www.miata.net/garage/tirecalcold.html Put in your current tire size and then play around with sizes on 17, 18, and 19 until you find one that has the same overall diameter. It doesn't have to be dead on, but within 1/2" will keep your speedometer correct and keep the same look, stance, etc. If you keep the same width and use a lower profile tire to keep the same overall diameter, then they're almost guaranteed not to rub.
Now, make sure that you visit some BMW 7-series forums or do an internet search to see what width of tire you can fit without rubbing.
Then make sure you choose the proper offset. If you're keeping the same width tires, keep the stock offset. If you want to go larger width, chances are you'll have to choose a different width/offset of wheel. Only experience and careful measuring will determine that. That's where the forums come in really handy. There is a 7 series forum here under the BMW heading.
With a car that heavy, be very cautious of going too big. Tires with small sidewalls have many drawbacks; unpredictable handling, less forward traction when you want to smash the pedal, and its incredibly easy to bend wheels. Little unexpected potholes on the road will turn your 18" investment into scrap. You're also going to add a good bit of unsprung weight to the party which can negatively affect handling, acceleration performance, and ride quality.
Keep the tire sidewall profile as thick as you can. The tires on my 17" have a 4.3" sidewall and three of my four wheels are bent. I don't know how the bling crowd puts up with it with their 24" chromies.
I think switching to 17 or 18 is physically well within reason. There should be no trouble at all if you're willing to put up with the negatives. Some people don't notice the negatives at all
You could switch out for the 17" BMW sport wheels. I think they came with 245 rubber and you know they would have the proper offset for your application.