The cabin collection sump is under that guest stateroom inner bunk. There is a sump pump and float switch in that sump. Is there water collected in there? If you are leaking water anywhere in the cabin, it will end up there. Also there are two water fittings right in front of where the water lines go past that sump. Those two fittings branch the hot and cold water off the main trunk and run to the galley sink. Those fittings on my boat were leaking and I had to replace them. There are no other fittings until you get to the shower sump in the hall. All the fittings for the two heads and the anchor locker are right there in front of the shower sump. There are lots of them there. Looks like you already checked all them.
IMO with all the stern weight in this boat, 400HP+ engines are needed to mantain an effective cruise. I would suggest you sea trial gas and different diesel options to see the firsthand differences. If you are doing any coastal runs in various seas, diesel is my only recommendation.
I am getting my dinette cushions and port sofa cushions redone with new padding. They had broken down to nothing. So I removed them to take to the shop. Just thought I'd pass on that the are all held in with screws from the underside. Forward dinette screws are accessed just by opening the doors underneath. Aft dinette cushion has two where you slide out the storage shelf and two up in that small hole in the second picture. Bring a long screwdriver for those two. You will see five screws up there but only the inner two hold down the cushion as I found out the hard way. Bench seat is easy. Just remove the skirt and six screws hold it in.
It doesn't separate from its tracks, but it pulls out far enough to clear the end of the cabinet and easily see and reach the two screws on that side. There is a vertical separation panel/support in the center of the compartment. So you can't get to the two portside screws through that end anyway. You have to remove the sofa seat and stick your shoulder/arm up into that hole to reach the other two screws.
I may know of one (400DB) but I am not familiar with a "test drive" for a boat someone is only thinking about. The process is usually do your research, find a suitable boat in the condition you want, make an offer, once accepted you get to ride on the sea trial. Throw a hull and engine survey in there and you are buying a boat. In a private sale you may get to ride on the boat before making an offer. If you are really serious about a diesel 400DB, send me a private message and I can give you some information...
