Happy Sunday to you too (from the "David" who keeps the cars for sale information up to date, or tries to!).
The Ford 4R75W box is very reliable. Not to say it can't break of course but generally, and is in my experience more reliable than the Jatco's fitted to the V6's, say; I've 'lost' two of those boxes over the last few years. Huge numbers of NY city cabs and cop cars used the Ford 4.6 2V modular back in the day, many were auto's too using the 4R70W or 4R75W box, and still run at over 400k miles.
My R75V8 is now just 200km shy of 350k kms (219k miles), and still runs like new on the original engine, transmission and diff.
Recently had a failed selector linkage ball-joint, which is not uncommon, but that cost only £10, and many bared knuckles, to resolve. The only power-train related problem it has ever had, as far as the fsh shows.
Keep looking at the availability lists I maintain starting with the Complete List here: https://www.two-sixties.co.uk/mgoc26...-260s-FOR-SALE . Updated at least 2x per day.
Tip: Rover 75 V8's are rare, but the Rover 75 V8 Connoisseur SE, VIN #605 belonging to member "Pat" in Kent, (listed on 01/02/22, 2004 Rover 75 V8 SE auto, Starlight Silver 91,650 miles £8,750), although officially withdrawn from sale, could still be available perhaps, if you ask. Try sending him a PM, maybe?
David