There are differences between the 66 models. But the base machine uses the same parts from the beginning to the end. From my memory there are minor differences in the bobbin carrier retainer then after the change they remained the same. Different bobbin winders, different hand wheels, but the treadles all have the big spoked wheel. And of course the different stitch length adjusters.
If you are restricted to just one, then I'd suggest getting the machine in the best mechanical condition.
I started out with 1 Singer 66 treadle that sat in the garage unused for 12 years. It was a gift from my late cousin.
Then within 3 months of getting it out and starting to use it I have 4 treadle machines and two electric cabinet machines and I've lost count of all the portables. Addicting is an understatement.
Joe