By observation, a parts car CAN be a good idea, in general. Provided you can store it in such a manner as to keep any municipality operatives in Code Compliance at bay in the process. Much less any HOA operatives, if applicable.
The other consideration is to get the same model year as you have, so that you don't encounter any changes from one model year to the next. Chasing additional hook-up parts can be an expensive (time and money) bummer, too. This might work better for the rear axle (if needed) than the trans, possibly. Do NOT presume that "they're all the same", although they might be. And then there's "the search" for a vehicle which was not a generally high volume seller, back then. The "joys" of owning/driving something different as an only car, unfortunately.
In some respects, I HOPE what you were feeling was a "judder" from the trans frictions not working smoothly. At least the Low-Reverse band is still working decently well (why it can move in "1" but not "D" or "2"). That modest-rpm "neutral drop" would be the same a popping the clutch on a manual trans car at the same rpm, by comparison. Normally, those activities are "surviveable", but if things are worn and on the verge of failure, they get there sooner, by observation.
It might be an option to "cheap-out" the trans rebuild, but with that much failure, building a good core trans might be better. Which is where the quality trans shop comes into play. No need to make it a bang-shifter, but keeping and enhancing the factory quick/firm shift orientations in the process can work well.
I believe that many in here have been in similar "When it rains, it pours" situations. Somehow, we all got through them and lived to tell about them. You can too. Whether it's vehicle issues, family issues, housing, or whatever issues. Let your "engine rpm and oil pressure" come back to normal and formulate a plan that is executable and then move in that direction. "Good choices" can be important. Best of luck!
Take care and please keep us posted on your progress,
CBODY67