I've been using an e30 (a 325ix) as a winter beater in Minnesota for about ten years now. It was rusty when I bought it, and while the rust has gotten worse over the years considering how infrequently it gets washed I'm surprised it's not worse than it is.
They'll rust out in the bottom seam of the doors, the floorpan around the jacking points (particularly on the driver's side), at the bottom edge of the front fenders where they bolt to the unibody, in the rear edge of the trunk lid and around the rear license plate lights. Mine also has rust in the sunroof, which from what I've seen isn't all that unusual either. If you're not too concerned about cosmetics none of those things are going to leave you stranded on the side of the road.