I also have a Hinterhoeller Niagara 26. Ours is hull 002 built in 1979. There is not much available on the net about this model ... especially inside pics.
There is the start of a web site at www.niagara26.org. The creator of this site has his email address on the page and belongs to a club that has an active class of 10+ Niagara 26 boats.
It seems to be a good boat but I guess you can only say that if you owned one for about 5 or more years.