Hi Brian,
I would try the University of Glasgow Archive service, the Ballast Trust, and the Mitchell Library in Glasgow if I was searching for plans...maybe others will have more suggestions. I have dealt with all three in the past and they were very helpful.
The book "The Caledonian Canal, Lochs, Locks and Pleasure Steamers", by Guthrie Hutton, has quite a few good photos (15 or so) of GONDOLIER including close ups/on deck shots, none really side on though. Would be good for model detail.
ISBN 1-84033-033-3.
This sounds like an interesting project and I hope your research is successful, let us know how you get on, it would be nice to see the model progress.
PS
what Duchess of Hamilton did you model, was it the paddler of 1890, or the 30's turbine? Would be great if you could post some pics of her.
regards,
Clark