Low Mileage Oldsmobile Wagon (78,616mi)

My daily driver for almost 2 years, which has recently encountered some sort of **mechanical problem, rendering it's drivability unknown. I'm the 3rd owner. The last owner was a mechanic who bought the car from his grandmother, so it hasn't traded hands very much. The title's clear, and I have the service reports of the work i've gotten done to it.
The Good:
-low mileage (78,616)-it gets 25mpg, so it's a decent gas saver
-interior's in great condition
-no detectable rust
-all power windows, lights, and locks work
-comes with two sets of keys, half a tank of gas, and the original radio
-(subjectively) it's a really cool looking car. Back to the Future meets suburban 90s mom?
The Bad:
-the mechanical problem
-1 of the 4 floor mats is missing
-the rear hatch door struts are worn out, so the door has to be propped up
-a few scratches and scuffs on the body, but nothing too noticeable
-the passenger sideview mirror doesn't stay in place (the mirror itself, not the whole piece)
-3 doors leak, so the weatherstripping should be replaced (luckily for you, i have the parts)
-the A/C makes the engine whine, but the economy air + heater options have served me well for a long time
-the key is broken inside the ignition, so the car has to be turned on and off via fuses and a screwdriver
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**The problem: I've been trying to figure it out for awhile, and having asked around a bit, these are the main hypotheses:
1. The catalytic converter may have come apart internally and is now clogging the exhaust.
2. The torque converter may have come apart internally
Something in the engine started rattling one day, but it didn't affect how it drove. Eventually though (in a progression of about a week), the rattling got worse, especially when I tried to go faster than 45mph. Then, on a cold night, I backed down my street, shifted from reverse to drive, and it wouldn't go when I pressed on the gas (I only tried once so as not to cause further damage). So I turned it off, pushed it home, and it's been sitting since.
In the end, I don't really know what's wrong with it, but you (or you and your mechanic of choice) are free to come check it out for yourself anytime.
It might start and run, or it might not after sitting for so long (about 4 months). You'll need jumper cables or a tow truck.
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Sidenotes:
-As you can see in the photos, the car's splotched up with a variety of paints. I used to do abstract paintings, and I liked to paint the leftovers on the car. But don't worry, it's all acrylic.
-There's a built-in box in the back to lock up your valuables (keys included)
-Cool backward-facing rear seat is cool.
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This is a project car. It obviously needs work to be reliable and frustration-free again, so just keep that in mind.
Any questions? More pictures? Message me.
NOTE: A deposit of $15 via Paypal is required post-bidding to determine the seriousness of the winner. I've been ghosted twice after successfully selling the car. This costs me my very few free vehicle listings, wastes my time, and drives away other interested buyers, so please only bid if you're committed to purchasing the car.