Jaffa,
how long is a piece of string?
Or more seriously, what do you want to turn?
Pens & Bobbins need a small lathe,
Bowls & Platters (especially large ones), need a powerful motor to work against the torque created by the tool.
I'm (generally) a spindle turner, I need a long bed as opposed to a large swing over the bars.
A good general purpose lathe will have a 1kW motor, 12 to 14" swing and 28" to 30" bed length, but even this varies with manufacturers.