I can only speak to my own experience. I was also gifted a car from my grandmother when she gave up her license.
The most expensive parts of owning a car for me are insurance and maintenance.
My car was a 1999 Chevrolet Metro. To put that into perspective, it has easily 1/10 the electronics your bimmer would have, and also 1/4 the powered systems under the hood. It only had 88,000 kms. Simpler, cheaper.
The problem is, cars age through both time and kilometres- some items wear out chemically speaking (mostly rubber, but some lubricants, etc.)
If you get her to send you the VIN (vehicle identification number), you can start doing some research online- If you check out your public library, they may have access to a maintenance manual database you could use to get ideas of costs for the maintenance schedule. For example, mine lists the jobs that would need to be done and the cost in person-hours at local mechanic rates ($120/hr CAD in my area.)
To replace all the parts that had degraded by virtue of time, I spent a grand total of $2800 CAD. Essentially, some rubber hoses, easily replacable spark plug wires, spark plugs, 2 belts, a water pump, a gasket, every fluid imaginable (coolant, oil, transmission 'fluid' (another oil), filters (air, oil, tran fluid) and brake fluid. All which need to be done on various schedules based on clicks and years. The 'unexpected' expenses were a full brake line replacement, a catalytic converter, and a pair of oxygen sensors- all 100% by the book. The upside is that properly replacing all the fluids on time/oiling what needs to be oiled can go a *long* way in making your car last to the proper 350,000 kms+ that I would expect to be the lifespan of a reasonably maintained modern car.
That was doing most of the work myself, by the way- with the mechanic taking $760 in labour for inspection and a brake line repair I was too scared to attempt on my own.
Doing most of the work myself, I have spent 1/2 my entire discretionary budget from last year on this particular car.
Now, that being said, the only maintenance needed now for the next 2 years are oil changes ($23 CAD a pop) and the occasional air filter ($10 CAD). Then it's back to replacing brake fluid, coolant, possibly automatic transmission fluid, etc...
Basically, Get your VIN, call a few local insurance companies, find out what it would cost to register it, see if you get a tax break if she 'gifts' it to you (otherwise you're on the hook for purchase taxes (?) and then run the numbers.
I'll tell you that I drive a tin can compared to your bimmer, and I spent 3x on it what I spent on uber/transit/car rentals for intercity trips combined.
Other (seriously mechanically inclined) mustachians on this forum have been known to keep costs of their car down to about $0.10 USD/km- $0.21 USD/KM.
My car is sitting at about $0.50 CAD/KM at the average, but if I managed to get another 40,000 kms out of it with no further heavy incidents, I should be able to get that number down to ~$0.24CAD/km (gas and insurance only.) Depreciation is a hell of a thing.
... It's also possible to "Ride dirty" and ignore all non-catastrophic issues with the car, only changing oil and brake pads as needed, ignoring the rest of the maintenance schedule until the thing literally falls apart around you or (more likely) just doesn't start one day.
Cheaper in the short run, but likely to slaughter your car by 160,000 kms or require pricey repairs (transmission replacements, radiator replacements, etc.)