Dave
I think you're approaching this from the wrong angle. It all depends on the size of the real thing. As I've been known to point out, a 1/96 scale Titanic would barely fit into my workshop whereas a 1/96 coracle will happily float in an egg-cup.
Start with the approximate size you want the finished model to be, then work out the exact scale of that in relation to the full-size. Once you've got that value you can look around for fittings and figures at the nearest available "common" scale and recalculate the model size to fit exactly – or decide whether the difference is significant.
Take the Calmac ferry 'MV Lochinvar' as an example. It's 43.5m long = 145ft. Let's say you want the model to be around 4ft long, so that scale would be almost exactly 1/36. However there are quite a lot of 1/32 scale figures and fittings available, and re-calculating the size of your model to 1/32 to suit them would make it just over 4.5ft. On the other hand a six-foot man at 1/32 scale is only a quarter of an inch taller than at 1/36 scale – are you bothered?
Drafting from photos has its own set of problems, the biggest of which is finding enough large, clear and relevant photos to permit you to draft a reasonably accurate shape. If you can access the real thing and take your own photos with a three-foot rule in the pictures then that is ideal. Once you're happy with the shape and proportions of your sketches then you can re-scale them to suit the required model size.
Dave M