Tom and Bess,
You all have talked yourselves into that boat. Pretty much you better decide what you can negotiate.
When I was looking and yapping with diesel mechanics, I consistently heard that Volvos were "high-winders" that their RPMs at cruise were higher than other engines, and may have a "shorter" life cycle. Basic mechanics doesn't scare you, Tom (like it does me) so I shouldn't think that the normal exterior stuff like heat exchangers, piping, and injectors should be a labor cost as much as it is a parts cost. Not sure the hours on the engines (?) or the make and hours on the generator (?). Generators can also be costly, so inspect it closely.
90% of the problems a boat can have, I've seen you guys fix, so you should be fine. This next boat will be way more thirstier ($) than Skinny Dippin'. Be ready ($). Fuel is down from a few years ago, however