The OBD II specification says that the scan tool you get for your car MUST work on any 1996 production or later vehicle sold in the USA. The scan tool for your BMW will work on your Mom's Chevy, if the Chevy is a 1996 or later model year. It will work on anything starting from the 1996 model year.
The data port for the OBD II scan tool is located along the bottom of the dashboard in the area of the driver's legs. Typically the port is located generally over the clutch pedal area (whether or not there is a clutch pedal), but I found a Suzuki that had the data port on the driver's side of the center console, about over the gas pedal. I've heard that Acura cars might have the data port behind the ashtray.
The Scan Tool will accept any of the data schemes -- there are different ways to present the data, and the tool has to be able to accomodate each of the ways. The data port is a standardized size and shape so the connector from any scan tool will fit any car.
I don't think the average Joe like you and I should need a $300 scan tool, but I encourage you to get more than a $50 scan tool. My scanner ran about $150, and it tells me most of what I need to fix my car. I was getting some strange readings that I didn't have enough information to confirm, but it turns out that what I though was the problem turned out to be correct so I did have the information I was looking for. I'd suggest without reservation that you should have a scan tool that ran from $120 to $150. Spend more if you want, spend less at your peril.