Hi PDX. I can't answer all of your questions but I can hopefully give you a little information having just completed the purchase of a NS30U.
First and foremost I LOVE my new boat. I have previously owned a C&C 24 and a Pearson 30. Both were great boats but not quite what I was looking for. Both were fast and pointed high. The P30 I put extensive work in to and loved that boat. My wife didn't. She wanted something more comfortable and easier to sail. Our searches brought us to the Nonsuch. We live on the Gulf Coast so it took us a while to find one here to look at. When we saw one on Yachtworld we drove two hours to look at it. Both of us fell in love immediately. So the research began.
The cored hull was a huge issue for me. It made and frankly still makes me nervous. Everything I read said Hinterhoeller did it right though. During my survey the surveyor paid extra attention to it and struggled to find anything that would indicate delamination or wet core. Trust me, her TRIED to find it. The boat had sat for nearly three years in the water. It was cleaned regularly but that was it. The keel/hull joint needed obvious work but I used that to negotiate the price down. Can coring be a problem? YEP! Just make sure it's checked thoroughly and checked regularly. From what I was told by the company that did my bottom work after purchase if you catch it early it's not a huge deal but if you don't it can trash the boat. Make sure no thru hulls have been added since it was built. From the factory I was told the areas where the thru hulls are are solid glass. Any new holes would have went through the coring though.
Check the mast and check it again. There were three different ways the two sections were joined over the years. The first two weren't that reliable and the flexing of the unstayed rig can cause cracks and eventually failure if not maintained. Do a google search for the International Nonsuch Association. Their website and discussion group offers a HUGE amount of info for these boats. You'll read until your eyes bleed at their discussion group. The INA's website has some PDFs about mast construction and what to watch out for.
My boat is the shoal draft version. It was found during the survey the keel/hull connection was in serious need of repair. Probably from a grounding. After purchase all of the keel nuts were pulled and replaced, the keel joint was ground out and floated with West System G Flex, then faired and the bottom sanded and repainted. It looks like new now and there is no movement by hand of the keel any longer.
Beyond that after sanding the bottom, a few small blisters were found. They weren't found on the survey because they were under layers of bottom paint. They were ground, dried and treated then a barrier coat was applied then new bottom paint.
Something else to look out for. I'm not sure all Nonsuch's had this but mine did. After the bottom work was completed we launched only to discover water leaking in from somewhere. At first we thought it was from a cockpit thru hull drain but we found it was coming from a stringer that helped support the rudder tube. It turned out that the boat originally had a stainless steel "rudder guard" that was hung in front of the rudder to deflect lines and such that might get ran over away from the rudder and prop. At some point in the boat's life it broke loose and rather than replacing or pulling the left over bits some previous owner just sanded the nub off flush with the hull and painted over it. It took a while for the yard to figure it out but over the years that had corroded and was allowing water in underneath the stringer. The wood was obviously swollen when the survey was done because we never saw the leak. The boat was out for over a month and a half and our guess is the wood dried out so when we relaunched the hole was opened back up. We had to grind out the left over corroded stainless, drill it all out then the yard laid up several layers of glass and epoxy to seal it back up. They then replaced the stringers and repainted the area. Leak gone and better than new.
In the end check all things you would normally check during a survey but for a Nonsuch pay extra close attention to the hull and the mast (along with the rigging that the boom hangs from). If that's all in good shape anything else can be repaired fairly simply and you'll have a hell of a boat. The INA is an excellent resource for more research as well.
Let me know if you have any particular questions. I'll do my best to help you answer them. I can't answer the B/D ratio. I've never sailed a 26 but the 30 is comfortable and anything from a beam reach and down you will fly. Upwind is a little tricky. Just remember to trim the sail like a Genoa and you'll be surprised how high she'll point even if you are going a knot slower that the sloops out there. You won't care since all you have to do to tack is turn the wheel while everyone else is fooling with sheets and winches.
Attached is a before and after collage of the bottom and a picture of the leak spot.