I would have to agree with Cracker as a suggestion. It's small and very simple, and the only thing that stopped mine coming to fruition quickly was the builder. If you start out with even half a clue you'll be fine.
The only part of your requirements that could be a problem is "…something that looks trainish…"
By this I'm inferring you mean "Mainline", which a cracker does not really invoke, however there are planty of examples dressed in different ways – You could fabricate some dummy side-tanks to hide the vertical cylinder and flywheel and get a look a bit like a dockyard shunter (squinting may be helpful).
I also think it could be scaled up to G1/45mm without too much trouble (There is a guy on the US-based MyLargeScale forum who has built a few G1 derivatives) and the short wheelbase means tight curves are not much of a problem (I've been running mine on a test oval of 32mm gauge track with 12 inch radius curves) BUT if you already have track or a running venue in mind, then this would obviously influence your gauge choice.
So long as you measure and drill the axle-holes accurately, there's not much to go wrong.
If you were to build it in 32mm/o-gauge, I would up the crankshaft gear to 12T from the plans 10T, as this enables you to use a 3mm crank-shaft (same as the axles); you can drill/ream a 10T to fit a 3mm shaft, but it's difficult to do accurately and leaves a very weak-looking gear.
I've posted this elsewhere, but this is mine (not quite finished) on a test run. When full of gas and water, it runs for about 12 minutes.