The fact that everything will have to be scratch built is a minor obstacle. Deciding on a scale might be a problem, but using Alan Butler's figures means any scale is possible. The big problem is the horses. A steam loco was tried in 1826, but was withdrawn rather quickly - usual problem of broken rails.
But the sight of an early steam loco, crossing a horse drawn train has a certain fascination, and will challenge the perceptions of some people who have fixed ideas of trains in Somerset!
Edit - here is a copy of the deposited plan, superimposed on the modern (1902!) map. Interesting that Mill Lane was built by the S&D.