Hello Exmoor,
The answer really revolves around several parameters, what skill do you have, you don't need to be a time served engineer, would it be your first project, what machinery, lathe, drill, mill etc; I've always built steam locomotives and if you fancy something like that, that isn't too taxing and will give years of pleasure then I'd suggest LBSC's Maisie, fairly straightforward, hundreds built, castings and materials readily available not to mention drawings and importantly a build manual, everything you would wish for and when completed they are easily driven and fired and on the track it will keep up with the big boys, they're a game little locomotive and will do all and a bit more on the track. I'd suggest you have a look on the Internet and youtube for LBSC's Maisie and you'll see a good number of examples. I would also suggest if you aren't already, join your local ME society, a very valuable resource and not to mention, a good supply of tea!
Regards,
Bob Youldon