Hello Richard,
It is always interesting when the question arises, what should the beginner start on, well the first thing is join a club or at least couple, look and listen, then you'll start to tease out what will suit you best. I've always said directly you start to make that first cut then you're in there with the rest of us, you're no longer a beginner.
I certainly subscribe to the idea of 3.5" gauge for your first locomotive, what you need is a well proven design without any vices, together with perhaps a written description, drawings and castings also need to be available, don't buy all the castings at once! In all of my many years as a miniature locomotive builder, perhaps too many! I think the locomotive that will always fit the bill is LBSC's Maisie, simple, uncomplicated, no difficult machining problems, straightforward and hundreds built over the years; a construction book is also readily available. I've seen all kinds of examples from the dog rough to cup winners, but they have all one thing in common, they go well.
If and when you do get started, build the bogie first, as one old chap said to me it'll give you something to push up and down (not on the dining room table) and it'll give you that extra impetus carry on.
Have a look at this chap enjoying himself: http://www.youtube.com/user/ianp6155?blend=6&ob=5
Also have a trawl around on the internet for LBSC, Maisie, there are plenty of photographs, links etc and if you are anywhere near the Bluebell Railway, they're building one 12" – 1ft much the same just the bits are a bit bigger!
Best of luck with your project.
Regards,
Bob Youldon