Welcome to the hobby.
Stationary engines are a good starting point for model building. With a small one, you have something to show for your labours, quite soon, which builds confidence. A traction engine can be a long term project, rather than a week's work!
But if you are not familiar with machining, make your mistakes, (We all do! ) on mild steel rather than an expensive and possibly difficult to replace, casting.
Getting to know the machine and what you can do with it can be by making simple tools.
Not only will you gain experience, but the tools will be useful for years to come.
Howard