Engine question ??

The 3406 is not the same as a 3196. If the3406's have been looked after ok they will be just fine with such low hours after 16 yrs.

As you only posted one lot of hours attributed to Cats it looked to me like you were under the impression the engines were the same.

If they survey well and have good maintenance records the 3406's should be fine.

I will not go as far as to say the same applies to the 3196 as I do not have any personal experience with them.

If your shopping for a boat you are going to keep for a long time, IMO the 3406s.
Lil more displacement for the same HP over the 3196. Sooner or later they all will require a MOH.
The Detroit has the least displacement but fires every time the piston comes up.
With the 2-stroke theory, that makes the most displacement per revolution.
Hard to believe the 3406 weighs more that the 6v92. 3196 the lightest.

Is the 3406 mechanical or electronic?

Most good shops can work on them all. Old Detroit marine techs may be getting harder to find.

There will always be parts available for all. Supply/demand will set the prices vs time. Forecasting who's parts will be cheaper would be an impossible guess.

How are the engine rooms laid out? Lots of service/work area?
When you do OH, got room?

Your probably comparing ZF, TD &/or Allison clutches. These would be large considerations also.

I'm finding some 56s in Florida listed with the 3408Es. Nice install with V drives.

All things being equal I'd take Cat 3406 over the others. 6V92 @2400 hours could be near the end. It depends on how they've been run and what hp rating. Typically the lower the hp rating the longer the service life.

Buying a boat with very low hours usually means it's been tied to the dock and you'll go through some gremlin busting once you start using it again.

In my experience DD6v92 life is from 3500-5000 hours - with not many getting to 5K. Cat 3406 I've seen go up to 20,000 hours or more. 3196 somewhere in between those two.

Again, a lot of variables though. Proper care reduces wear.

If you like producing coal and ramming the power through a bevel drive go for it.

Translation issue again but let me try on my own a bit;
I can only assume you're not a fan of V drives. There is a loss of power when going thru any gear. Reversing that same rotation is a large loss also. Parasitic HP loss is not parasitic any more.
Yes, your not going to make 800 HP and deliver 750 (good dream) to the prop.
I did miss type on the 3408. I am referring to the 3406 as I hope all understood.

Producing coal did not translate at all and I'm at a loss, WTF you talking about.

All things being equal, you can't go wrong with a 3406e, 3408 not so much.

My experience with 3408's and their ability to produce large amounts of poorly combusted rubbish which often takes on the appearance of black smoke is what my statement "producing coal" was referring to.

Get an engine survey. But the 3406 would be my first choice and may be ok even if engine hours are low. 3196s are good too and might be a better choice power to weight ratio. DD's 6v92's I haven't had issues with, but would be my last choice. I have managed a couple of 6v92 boats that didn't have any issues, but both boats were old and hours were under 1000. I'm managing a 1996 boat with 8v92 DDEC's right now, the motors have 1650 total hours, the boat is a 1996 and both motors were majored in 2003, then the port motor was majored again in 2014, and the starboard....well probably isn't very far behind it. I have never seen DDEC 92 series motors last more than 1000 hours between majors (personally).

Rolling Coal

Those are deliberate efforts, the good ole 3408 resisted all attempts to stop it. I was offered 3 on a new build as gensets. I politely declined and persuaded the Owner to upgrade to the C18 when it was a real new offering and he has never looked back.

Correct me if I am wrong.
The 3408 had a 60 degree V block and 90 degree offset ground crank.
A pretty durable engine.
It seemed to last well in a bulldozer at industrial ratings.
A fuel eater in a truck. 3 mpg at the time most other trucks were over 5.

I am not saying they were unreliable just undesirable when someone has to clean the soot and junk off the sides and or deck everytime the load changes or it starts.

Automotive applications don't normally start with the throttle set for 1500 or 1800 rpm so don't have as much problem with poorly combusted fuel thrown up the exhaust