^^Agree with you 4horses & people have to remember that any advice here is only general personal opinion, but OP is talking about passing the horse onto someone else because they can't put in what sounds necessary. That is why I'd personally consider putting it down, if there wasn't a good chance of keeping him at least paddock sound.
The 'navicular' doesn't necessarily worry me, as with good management this should at least be manageable, if not cureable, especially if he's only 5yo. I don't have much personal experience with advanced articular ringbone, but while the damage will always be there, horses can become 'sound' - well, pain free & rideable once the joint has completely seized & calcified. Can't help wondering about the rest of the horse too... So, assuming I had the means to look after the horse properly & provide the necessary meds & such, I would give it a good go, wouldn't put him down. But if I couldn't look after him, I wouldn't leave him suffering without treatment, or pass him on unless that was guarranteed, that the new owner was prepared to put in as much effort & money as it may take, for possibly nothing more than a short term paddock ornament.