I've just mounted a set of Heidenau K60's and one thing to be aware of is that they are more street oriented than the TKC 80. If you're planning to ride down to Morocco I'd recommend them although I obviously can't say anything about mileage yet.
However if you're planning to tackle serious dirt (more then say dirt/gravel roads) the Heidenau's might be a bit out of their depth.
How about sorting out a set of (say) MT21's (or other true dual purpose tires) in Spain beforehand, have them mounted there use them for Morocco and switch back after you trashed the live out of them. You might need to change down there anyway if you ride all the way. Highway miles will seriously wear down any knobby tire.
If you do really want tires that'll last as long as possible, check out the Mefo Explorer or Superexplorer, but you'll probably pay it with a slightly wooden feeling and decreased grip especially when wet.
Life is full of compromises