I don't think restoring classics and keeping older cars going is simply a 'worth it' type argument.
Owners do it for the love and passion they have about the cars. If they do go up in value even better.
With these 6.3's they are rare as you like so restoring them would be great.
People on here have spend some fortunes doing up older merc and keeping them going, even making them look better than they were new. Unlikley to ever see their money back but its more than that.