Welcome to the forums!
Running 18" or 19" isn't a problem for your car. As long as you run the correct offset, widths, and tires you wont have any kind of issues. Staggered or non-staggered could both be options for your E90. I would suggest taking a look around in the appearance cosmetics forum or vendor section to see what wheels there are out there.
If you need any kind of help with fitments, feel free to PM me.
-Charles@VMRWheels
www.vmrwheels.com