Murray,
Yech - that's a nasty question!! There are so many different types of lathes for so many different uses.
Personally, I'd like a small lathe (like a watchmakers lathe) or a little lathe that has a bed of about 200-300 mm, with a swing (the max radius that can turn on the chuck) of about 75-80mm. I'd be able to do much finer work than I ever could on a big lathe.
I currently use a 60 year old turret, technically not really a lathe, but we've fabricated a tool post with slides so it can be used as a lathe.
The turret is a lathe - sort of - that has a station of tools that can turn and be pushed into the work (stuck onto the chuck in the headstock) to perform a specific function - for example, the first tool moves in and drills a pilot hole then moves out, the turret turns and the next tool moves in and reams the hole then moves out, the turret turns and the third tool moves in and cuts a chamfer on the hole , etc etc etc..........
I'd recommend seeking out a group that either restores real live steam apparatus (like trains or tractors) or try to find a model engineering group who can give you first hand, personal instruction on the uses of lathes and milling machines. Making, repairing or maintaining live steam vehicles or apparatus requires extensive knowledge of machining and lathe work.
There should also be classes in lathe work and machining at your local community college that you might be able to join.
Or maybe, just have a chat with the manager of a local engineering shop, take some of your models with you so you can explain what you want, and I'd be surprised if the manager would'nt show you around and explain things.
I'd love to do that myself, but it's a long, wet walk to Canada.
Cheers mate
LeeTree