Definitely quite easy to drive the boat and handle with two. There are so many systems on a big boat though. And with an older boat that means so many systems ... each of which needs work! I couldn't imagine leaving mine for several months at a time ... it's the regular use that has allowed me to find all of the hidden issues. Once it's in perfect condition though, I suppose it could be left for periods of time.
The 10 cylinder MAN engines I believe were at one time referred to as the "ManGrenade". I believe there was an expensive fix for this. I have heard plenty of good things about the MAN engines ... one of them is the superior fuel efficiency over the old Detroit's which I have. The downside of the MAN's is the expensive service. So, like any Diesel, you will want them checked thoroughly by a trustworthy MAN mechanic. Of course, it would help a lot to know their complete service history ... but judging from the fact that you are asking about fuel burn, I am guessing you are not in touch with the previous owner? You should be able to get 2 to 3 gallons per mile at 20 knots.