Well, first of all without knowing the details of what exactly went wrong, $700 is a reasonable quote for a new boiler, element and control board plus labour to fit them. So, based on that Id say definitely get the machine repaired - if then you decide to sell it, you can recoup the costs plus a bit to go towards your new machine.
From my experience the Cuadras are built well, but Im not sure what internal revisions have been done over time. Maybe get in touch with Margaret at Queensland Coffee Machine Services (the La Nuova Era importers) and get a second opinion on whether there have been any significant design revisions.
Major component failures definitely do happen, more often with electronics in a hot humid environment, but the circumstances you describe are pretty weird. For the boiler to run dry and then do an element, the autofill circuit or pump would have to fail first, unless the leak was massive but then were talking catastrophic failure of a boiler seam which would have been a spectacular event.
More likely would be runaway heating due to a failed pressurestat, which was not caught quickly enough to prevent serious damage to boiler/element.
Any more details on events leading up to it no longer heating?