Difficult to go wrong with a full frame Canon or Nikon, but I think the early models of the 5d III had a problem with a light leak from the viewfinder, so may be good to check if this was fixed.
Also, mention that it's been only used for video suggests it's been used by a professional? So it may have had a hard life? Check for damage.
Main advantage of 'full frame' for portraits is more control over depth of field, but apsc isn't bad for this with the right lens. Imahe quality will be similar to most decent new 'non pro' bodies. For the same money you could get a nice new apsc body or some lenses.... just a thought.