Hi!
The first thing is that any boat will require upgrades and repairs. It's a feature of boat ownership that something will always need to be fixed/upgraded. It's just part of the price of admission to a season of being afloat.
I've not heard of any Osmosis issues for the Moody boats. They are well made, solid boats that will look after you. There's nothing especially problematic with the hull or engines that I'm aware of other than all the usual stuff around making sure the maintenance has been done. If there's a log book see how the engine has been used, talk to the owner about what they've been doing and so on. It all helps to give a good picture of the boat.
If the engine has been well maintained there is no reason for it not continue giving many more years of service. However, just keep in mind that at some point an engine swap may be needed due to the age of the boat (although in my last boat a 50 year old Hustler 30 I had a Volvo 2003 that lasted 35 years before it needed a head overhaul and it's still carrying on today!). So whilst there will be that future outlay at some point for engine work she'll give you many more years of great service after the work has been done.
There's still plenty of spare engine parts available and there's a LOT of support out there for the M50 engine and Moody's.
For us we wanted a go anywhere capable cruiser that will be just as much at home sailing in the UK as it would in the med or crossing the atlantic. These boats will do that with east BUT looking after the boat, will be a big part of the experience. Upgrading the boat so we can do things more efficiently and safely is just part of the fun of sailing for us. Boat repairs are just the way things are with sailing regardless of the age of the boat.
I looked at lots of boats before I settled on an M38. The Moody 38 just ticked all the boxes for me. The only ones that came close to the quality of the M38 was the Hallberg Rassy 382/352. However, between the HR and the Moody only the Moody has the awesome aft cabin
and this awesome Owners Association. The resources and knowledge here with these boats is superb... Very few used or even new boats have this size of support network.
Hope this helps a little and good luck!
Andy