Nordhavns marketing is US based and has done a much better job than Selenes offshore marketing people in presenting their boats. With that said, I know they are both excellent designs. Either one, properly maintained, will take you far. After carefully considering all the usual suspects the Selene had the right balance of what we were looking for, but we surely could do everything we wanted with either one.
You've been getting good advice so far. Between a used Selene and a Used Nordy it really comes down to how the current and previous owners have maintained them. I would be looking for detailed and extensive maintenance records coupled with frequent use. The lack of either, to me, would be an indication that it will cost a bit more to get everything back up and running. In my experience, with unused boats, gaskets dry out, heat exchangers rust from the inside, and these boats also tend to have zincs that may not be replaced as often as need be. Does that mean I'd walk away. Not necessarily. It becomes part of the price discussion if I'm willing to do the work or get it done.
Stabilizers are pretty robust items. Not much to go wrong, and all of it can be fixed without too much hassle. I'd be looking for how old the seals are. 3 years or more (at least for our Naiad's) and they'll need to be replaced. The hydraulic fluid should also be replaced on a schedule. But both are routine maintenance items. If you don't have to do it now you will soon anyway.
If you're looking at an engine that runs well I'm not sure why you think you'd have to rebuild it. One thing I learned, get an IR thermometer and during sea trail get the temp all along the valve cover. If any cylinder is 10 or 15 degrees higher than all the others there's an issue there you will need to address. At minimum it's a top end job, could be a total rebuild. Otherwise if she's running well in all other respects (not burning oil, no unburned fuel in exhaust, oil analysis ok, ect.) then you very likely have an engine that should continue to perform well for the foreseeable future.
Us, we bought a boat that we knew would need a top end job. We ended up rebuilding the whole engine. 19k if you're curious. Should be good to go for a long time now.
Whatever you do, I wish you the best of luck in your hunt!
