Hi
it depend on the condition of movement, bellow and ground glass, if they are all good, then €500 is reasonable. Its not a great deal but for a really good camera that will work for many many years its nor a bad deal either (but movement, bellow and ground glass should be perfect)
If you choose between the Sinar F (see if you cant find a Sinar C, it has a more stable/advanced back stand), and the Horsman LE i would go with the Sinar because you can get the Sinar shutter. Instead of mounting a shutter in your lens, you place the shutter behind the front lens stand. This mean that you can use really old lenses or lenses that has no shutter mounted. Of course you can also use lenses with shutters. For me this feature on the Sinar is one of the best, and you will never be sorry for getting a Sinar.
cheers