Motorplan and wheels

Hi guys.
I need a few questions cleared up.
I got my new car on tuesday, its a 2009 E90 320d LCI with 25000km on the clock :D

Now my problem is, its got horrible 16" wheels on, and Im looking at getting a set of 18" wheels. BMW said that it should be ok for me to put other wheels as long as they are 16,17,18' as I dont think the these models came out with 19". They were a bit skeptical about it and said if i do i should put the original wheels back on when i go for a service etc.
Now If i put 18" on with 225x40x18 and 245x35x18/255x35x18 will it pick it up when they put it on diagnostics, I was just wondering as i heard there are sensors for the wheels, as it has run flats. What is the widest i can go at rear on the E90. Will the motorplan be affected if i put other wheels on?

Also if anyone knows where I can get decent narrow and wide 18" wheels in Cape Town, as i have literally been everywhere and hardly any places have anything decent and at a good price.

Any other info and things i need to know would be much appreciated