Small metal Lathes / IE 350 to 50W machines

[bikepete]

Of those three the second (Star) lathe looks by far the nicest - most pricey too of course.

The first on your list is a generic Chinese hobby lathe - 125 kg, so a step up from a C3 but still no comparison to the big boys in the other links. Available from several sellers no doubt, for much the same price. For just a bit more money you could get the (heavier) Arc version with tooling bundle:

Model Super C6 Lathe - Arc Euro Trade

but it's twice your budget anyway...

You may be pushing it to get anything decent for 500 euros - maybe a plain turning lathe (without screwcutting capacity) perhaps? Or something very knackered. Or you might have to just get lucky (personal connections/putting word around locally might find you the best deal - I'm sure most sales don't get advertised.)