Hi Graham;
Questions you have to answer:
1) how will you move it about?
2) do you want to pull passengers all day, or just go and have fun for an hour?
3) Do you want a prototype model, or something freelance?
4) Do you want to have a fixit locomotive, or one that is reliable?
5) What gauge(s) are you interested in?
—-
My answers, ONLY for your contemplation:
1) Car boot, lifting. I live in a cold climate, so the locomotives get lifted into the house. Contemplating my larger one, and which neighbours to ask this time; 120lbs up the stairs and onto a stand in the living room gets a bit of a bother year after year; 2 locomotives under completion; 50lbs or less for each of them.
2) I like having fun for an hour. I love talking to people, but find pulling passengers around puts you as a slave to them, not their equal. (IMHO)
3) I do like proper prototypes, but my favourite – my 3-1/2" gauge Tich is (obviously) not.
4) Now, I'd rather make my own mistakes than find and fix someone elses. No more "fixer-uppers" for me.
5) 3-1/2" gauge, mainly. Where I live, 4-3/4 is the popular one size up gauge, but the local club has 5". 3-1/2" is actually a great gauge for me in many respects.
Make up your own answers to these and your own questions – and, there's no wrong answer.
John.