Hi Eric
I am just finished with some pre-launch work on my new project boat - 1980 Niagara 35 - although I remain a dedicated and long term Nonsuch owner/sailor. This project boat is also designed by Mark Ellis and built by Hinterhoeller and construction-wise has many similarities. We did make some repairs to soft areas in the balsa cored hull, below the waterline. Intimidating at first, but actually very easy to do and makes a good DIY project.
First, ensure that you have had a bona fide moisture test, and hammer-tapped the hull to be confident you actually have a problem. You can watch YOUTUBE videos on the subject of interpreting hammer taps, understanding rotten cores and sandwich delamination issues, etc.
You can drill some small holes in the suspect area (easy to repair) to inspecty and validate core condition. Removal of rotted core is mandatory, drying out adjacent core areas is mandatory, there are chemicals (OPES epoxy sealant, e.g.) that can help with this.
For our repair, we replaced the rotted balsa core with solid epoxy + microbaloon filler, not more balsa. Not large enough area to make a weight difference, easier to do in our case, more permanent for sure.
If you have an otherwise good purchase price deal on this boat, do assess the damage and cost to fix professionally, and consider that cost in your offer. This is an easy repair as DIY or by a yard, do not be discouraged by issues related to the repair which will be straight-forward.
Your ultimate reward will be sailing on, and enjoying the amenities of your Nonsuch.
Greg Silver
Misty Cat N26C #121
St. Peter’s, Cape Breton