As Chris says, your choice of lathe will be controlled, even if only partially, by what you intend to machine on it.
You would have difficulty making a quarter scale Traction Engine using a watchmaker's lathe, and equally well making watches on a machine capable of swinging 18".
You can either start with a small machine and as you gain experience, and your horizons expand move to a larger machine, possibly in more than one step. Or if you think carefully enough, buy a machine that you will keep for a lifetime.
I am not a Model Maker, but started with an at least secondhand Myford ML7 (Fantastic work has been, and is being done on them and their successors ) but I found it restrictive, in some respects, so changed to a larger lathe, which will swing 12" over the bed or 18" in the gap. From the sublime to the "gor blimey", some might say.
But it has been with me for longer than the ML7, and will see me out.
You need to consider, New or Used? There are lathes a century old producing marvellous work in skilled hands, and brand new lathes making rubbish for the careless and unskilled..
Used machines are good if you know either someone who knows what to look for and assess, or what you are doing. Get it wrong and you can have an expensive doorstop, or a job for life bringing it back into shape..
A starting point might be to try to decide what you want to make, and the features that think are essential to perform then operations that you envisage. Some folk manage very nicely without powered feeds or screwcutting facilities. Others think that they are absolutely mandatory.
before making any buying decisions, look at th various "What lathe shall I buy" threads on here.
And when you think that you are close to a decision, look at the Lathes UK website. This contains masses of info on many lathes. And read the posts on here about more recent lathes. There you may find details of weak points, and sometimes how they can be overcome. A chance to learn from the experiences of others.
One other bit of advice, allow budget for tooling and measuring equipment as well as accessories for the lathe.
Good Luck!
Howard