Soot black oil is normal. Drive around the block after an oil change and it'll get that black.

These systems aren't optimized for R134a. If you know your way around AC systems, you can probably revert to R12. Otherwise I'd hold out and even pay more for a car with suitable AC.

I'm rust ignorant like you but I'd pay close attention to the junction of glass, trim and body along the forward edge of the trunk lid. Once the seal gives, you'll have puddles in the pocket behind the rear wheel then puddles in the rear floor.

I don't know about glass fogging from water leaks. I know the glass they use in these cars can delaminate and there's nothing you can do about it other than replace the gla$$.

It's not difficult to tune transmission shifts but hopefully it's in fair shape that tuning will net a long term benefit. I'd go by the condition of the fluid. Wipe the dipstick on a white lintfree cloth and see if there's anything but red fluid. I'll accept a trace of silt.

2 tablespoons after a test drive is unacceptable. It might be easy to fix but that's a lot in relative terms. I've had engines with not a dry spot on the underside yet nothing makes it to my driveway.

Sixto
87 300D