I think if the neutral drain device fails then you would just remove it and it would work the same as a 500 series - slow spinning as it drains, speeding up as the tub empties. Nothing to worry about.
The transmissions are pretty reliable, up to 700 series the top agitator shaft seal is above the usual water level which is good design. Later (800? 900?) ones had a shorter agitator shaft, like a Maytag, with the seal below water level so if the seal fails, the transmission fills with water. Silly.
These are old machines now, transmissions could be simply tired and worn out by now, but you have options - more machines will turn up in future, you get others for spare parts. Also John L (Combo52) above has kindly mentioned a big stash of Blackstone parts in the USA, so if needed, you could order from overseas. I may have a part or two - pumps, both belt drive and electric, almost for sure. Not much though, I only ever owned one. (a 720.) These big top loaders aren't really "My Thing" as I'm much more into oddball rare stuff and front loaders, but I was always impressed with the Hoover 720. I also had a friend who was a mum of 2, she had a 620, knew it was old but swore never to get rid of it, if it could be kept alive. She loved it.