Welcome to the Forum!
A vast fund of experience hiding here, on all sorts of subjects.
Once you have defined your vision of what you want to make (Could be anything, A locomotive of one of the standard gauges, Ditto Traction /Road engine, Stationary steam engine, stationary I C engine, workshop tooling or whatever)
The scale / size that you choose will have an effect on what machines and tooling you buy.
As Noel says, don’t rush. Make yourself familiar with the machines and your and their capabilities.
Make you mistakes (You will, as you learn) on relatively cheap and easily replaced material.
A scrap bit of mild steel will be cheaper and easier to replace than some part of a locomotive kit, in both cost and time.
But you will have a lot of enjoyment and job satisfaction along the way. AND you will discover lots of jobs that you can do, or things that can be made, in the process, discovering just how versatile a machine tool can be.
Things that used to be damaged beyond repair, or weak designs, can now be repaired/improved beyond the original.
The world is about to become your oyster!
Howard