Uhh....I don't believe I ever said that all of the small motors are Tohatsus, just that a bunch of them are (Mercury, Mariner, Nissan and others have all at one time or another recently sold re-badged Tohatsus for their little ones). I guess it depends on your view of how many it takes to constitute a bunch. In any case, that 2.5 Suzuki looks like a nice little motor. We had a 4hp Suzuki on our Trimaran and it was quite dependable.
As to the proper motor for that particular canoe, I think the first logical thing to do would be to contact the boatbuilder and since they have most likely tried the boat, I'd ask them what size and/or brand of motor they used and found to work well. It makes a lot more sense than trolling the internet getting opinions from strangers who have never been in or even seen one of those canoes (including me).
Opinions seem to be all over the place with respect to the exact best location of the plate with respect to the bottom of the transom. A few I saw suggested slightly above the transom's bottom, most were about even with it and some were slightly below it. I suspect the more important question would be the plate's position with respect to the surface of the water, as it might make a bigger difference than the exact transom bottom level - especially on a canoe, where the rather small transom has far less drag and creates less disruption of the water than a typical, full-width powerboat transom. The plate's job is to prevent the prop from sucking air down from above the water's surface and ventillating the prop (causing it to lose power). That's why I think the relationship between the plate (and the prop below it) and the water's surface is probably more critical than it's relationship to the bottom of a small canoe transom.
If our canoe has a 17" transom and draws 3"-4", which is typical for most canoes but depends on the load, then we would have 13"-14" of transom sticking above the water level at displacement speeds. A 15" shaft would put the plate 1"-2" below the surface, which most folks seem to think is about right.
However, if your canoe gets up on a plane, the transom rides higher and the plate does as well. Some of the motorboat guys say that the plate should be at the surface when the boat is up on a plane, but I'm not much of a motorboater, so I don't know whether that's right or not. I've only truly planed reecreational-sized canoes with motors (oversized) a couple times and I found it to be rather scary and backed the motors down to a sort of semi-planing speed.
Anyway, that's my take on the subject and I wouldn't really want to have to draw any conclusions from it at this point. If I was interested in the boat, I doubt there is a better source of information on what has been tried and works well than the builder.