Rust under the front lift points (behind the front wheel well. Rust above the rear license plate, around the plate lights. Rust on the rockers just ahead of the rear wheels. Rust in the battery tray area or the other side of the trunk where the jack goes. Bottom of doors. Sunroof rust - 70 - 100$ for a used replacement.
Another good thing to do is smell the carpet, and feel the carpet, if you feel moisture, you are almost guaranteed to have rust/holes under there.
The Rockers and the front lift points can be car killers, simply because people can't afford or don't have the time or tools to do it themselves. Skip these cars unless for parts, or if it's an extremely rare model.
If you see surface rust on the fenders hood or little dents here or there it's nothing to worry about, make note of them for adjusting the price but.
Remove the oil cap, check for a greenish tinged crud. A little white goo isn't bad that happens. remove coolant overflow cap, look for oil floating or at the bottom of the overflow container.
Run the car. note idle stability between 1100 and 750... depending on the temps.
Listen to the engine, the ticking is normal but listen for any offbeat ticks or knocks.
If it's manual shift in in and out of each gear, many linkages are old so if it's really wobbly you may need to put some money into it, just make sure it shifts.
Automatic - pray to the bimmer gods.
Check for brake pressure. From the outside of the car check the brake disc surfaces for heavy scarring or uneven wear, could mean a bad caliper.
Check all the power windows and the sunroof.
I could go on but generally if it drives at least as well as an older car, and there isn't rust holes in the rockers or the front jack points, then consider buying it.
Performance one step at a time.