Cabo Yachts

That's what I've got on my boat.
V8/800 engines are the very last of the old school MANs, 100% mechanical.
Solid stuff, almost as good as the newer common rail engines but without the annoying occasional sensor faults, hours count going nuts, etc. associated with them.
What gearbox are they mated to?
ZF350 was the most popular combo, which according to a ZF chief engineer is one of their best (if not their absolute best) gearboxes ever.

Usual maintenance aside, you should check the following:
- injectors should be pulled and tested every thousand hours (though once you pull them out, it's also worth replacing their nozzles even if still in decent conditions, for not much more money - that's what I did, anyway);
- heat exchangers and aftercoolers should be cleaned every 2 or 3 years/400 hours, though to some extent that also depends on where the boat is being used. Check also that there's no leaks around the side round covers of the HE, particularly those on the left side.
- valves should be registered every 400/500 hours.
- last but not least, it's worth mentioning that the aftercooler is rather peculiar. In a nutshell, it's split into two sections: one of them feeded by the close cooling liquid, and the other by raw water. But the latter, which is the most important, is controlled by a valve that allows raw water to circulate only when sensing turbo pressure. In other words, the aftercooler is designed to actually warm rather than cool the air, when the engine is spinning at low rpm/load (in order to optimize combustion and reduce smoke). Which is all well and good, but only as long as that pressure-sensitive valve is not stuck, otherwise air is not cooled as it should, north of 1400rpm or so, which is when the aftercooler really matters.

Should you go ahead and seatrial the boat, I for one would be interested to hear your impressions on the powerplant.
Good luck!

I have 12 years with V10 820s and would add the following. I had to replace the torsional couplers on mine (25K for both Labor and materials). One went apart offshore of course. I think most on this forum find my maintenance regimen extreme but it has served me well. That being said I would budget to send the fuel pumps off for a reconditioning (8K ea if nothing is wrong). My experience leads me to believe turbo reconditioning could be in the somewhat near future. I had mine blueprinted and boost went up and EGT went down. Also had my injector lines replaced as well. I do my 1000 hr service every 3 years. I never hear that the heat exchanger looks clogged or scaled but I am told the real reason to do this every 3 years or so is to lubricate where some dissimilar metals touch. I might not have that exactly right. Mapism/others will correct me. Mapism’s knowledge far exceeds mine but I had the ZF 350’s and one of mine cracked its case (no hard strikes). The 350s are no longer made so I had both changed to 360s. I was told that the 350s were not well thought of but that is just my experience so perhaps my experience there is an outlier. Lastly the Mangrenade thing has been way overdone. MANs are just like all engines manufacturers in that they have some good ones and some crappy ones. Yes they are expensive to maintain but they have been very reliable for me over my ownership. Good luck and don’t be scared of the MANs.

MYlover, if you don't mind me asking, what ballpark hours had you clocked by the time you had:
1) the torsional damper falling apart?
2) HP pumps and turbos needing reconditioning?
3) the ZF350 cracked?

1 & 2 sound like pretty high hours items, and Scott Roush only mentioned the year but not the engines usage, in the boat he's considering.

Ref. the gearbox instead, I am sorry and surprised to hear of the problem you experienced.
Even if your V10s are a bit more powerful and torquey than the V8s, they are still well within the 900hp @ 2300rpm the ZF350 are rated for.
Did you possibly run the engines under heavy load at less than the max rated rpm?
The reason why I am asking is that the output of your engines is supposed to be in the 500 to 600hp ballpark, when spinning at 2000rpm at the typical prop demand curve, but they can actually produce almost 800 hp already starting from 2000 rpm, if that's what the boat (hence the props) demand. That can be due to fouling, heavily loaded boat, sea conditions, whatever.
Now, while this is still within the engines capabilities and therefore "tolerable", at least as long as the engines aren't kept running forever in these conditions, it can quickly become critical for the gearbox.
In fact, the same gearbox rated for 900hp at 2300 rpm forever could well be unable to sustain 800hp or so at 2000 rpm, for more than a very short time.
Not saying that this is what happened to your gearbox, of course.
I don't think there's a way to know for sure, as I suppose also your mechanic told you. I'm just trying an educated guess.
It could also have been a defective case, though that would have more likely surfaced relatively soon.
And since you say that the gearbox was already phased out by the time it happened, I suppose it was after some years (and hours).