It ultimately depends upon budget.
As for a ND Sub, I would go 5513 rather than pay the premium for a 4 liner 5512 (which are harder to find and hence more expensive). The meter first or Maxi versions are super nice.
Personally I would avoid a 1680 unless it's a Red so would look for a 16800 instead as it has a better movement and quickset date. The downside is you will not a domed plexy and will have to put up with a sapphire crystal.
If it's a GMT that tickles your fancy, again I would avoid the 1675 and go straight to the 16750 because like the 16800 it has a better movement and quickset date. However unlike the 16800 the 16750 retained the domed plexy and looks much pretty identical to the 1675 especially if it has the earlier matt dial.
But if you have the budget, the best vintage Rolex from this period has to be 1665 SeaDweller.
iMHO of course.
As for replacement bits, it's best you find a watch and get expert advice, simply because these watches are at least 35 years old and chances are that something might have been replaced at some point.