I've had an ex ORS F650 Dakar and there was no problems with it whatsoever.
Mate currently has an ex ORS XCountry and, other than the obvious dents and scratches, mechanically spot on.
Tatters_2005's XChallenge looks a good buy. I paid £1200+ more for mine from a private bod and, whilst it was mint, would query what extra 'value' the extra cost added, particularly if you end up trail riding on it anyway.
All the above have the low maintainance F650 based engines though, and are renowned for being robust. I would be more careful re the higher stressed bikes, such as the G450x range.