MS350/390 pre-purchase exhaust inspection
As nice as it sounds, tearing engines down for a pre purchase survey is not common. I wouldn't allow an unknown wrencher to do anything on my vessel during a survey except to take oil samples - and often these are done wrong.
A good engine guy (and many surveyors) can spot a haywire exhaust riser layout in a second while noting other mechanical visuals. Coupled with sea trial and a review of maintenance records that is largely the mechanical inspection.
Any problems found can then become a further discussion.
Re:
"As nice as it sounds, tearing engines down for a pre purchase survey is not common."
I couldn't disagree more. For a marine diesel application, disconnecting the wet exhaust elbow from the turbocharger to inspect the exhaust side for water intrusion is not 'tearing down' the engine. Rather, this is a normal and required periodic inspection task that only takes 15-20 minutes (after you've done it a few times). Both of the saltwater turbo-diesel-powered boats I purchased in the last five years (with engine surveys) had the wet-elbow disconnected and the inside of the turbo housing photographed before I accepted the vessels. On both my boats, I do this inspection at least annually (1x Yanmar 6LYA and 2x Cummins 6BTA).
Re:
"A good engine guy (and many surveyors) can spot a haywire exhaust riser layout in a second while noting other mechanical visuals."
A good engine guy will tell you it is not possible to look at an exhaust system from the outside to tell whether water has gotten into the turbo, until it's too late. By the time you can see any evidence of damage from outside, the turbo (and maybe the aft-most exhaust valves) will be toast.
With the Mainship 350/390's this is an especially critical inspection. One of the biggest 'known issues' with these boats is their history of taking water up the exhaust and flooding the turbo and ruining an engine. Our 2003 MS390 has a 2007 engine for
exactly this reason, and as of 2007, it also has the latest (final) redesigned exhaust system -- a new exhaust riser and check-valve setup.
Having this upgrade is one of the biggest reasons we chose this particular boat in 2014.
Even with the redesigned exhaust system, there is absolutely no way to know if there has been salt-water in the turbo without doing this inspection. This is especially critical after a big storm, if the boat was in a less-than-fully-protected marina.
The other (equally important) reason for doing this inspection is to monitor the condition of the wet elbow itself. These elbows (even the stainless steel one on our 390) are subject to corrosion, and once they start leaking, you will start getting salt-water into the turbo.
This is yet another condition that cannot be seen from the outside until it is too late.
I would never buy any turbocharged marine-diesel powered boat without seeing the inside of the turbo. If the seller objected, I would walk away. In my case, neither seller objected -- indeed they allowed (and expected) the inspection, although I had to pay for new exhaust gaskets).
Rendern was the one that said RUN. I apologize. 
