Brake calipers frozen vs leaking, rebuild kit needed?
 

For the most part, yes, correct. When frozen there can be damage to the piston - corrosion or otherwise. It would be difficult to clean the piston and still make a nice seal.

Leaking is just a seal problem, so easy to resolve.

Honestly given the price of rebuilds, I went that route. Sure, costs more, but then its just a swap and go. And if you run into issues trying to rebuild on your own, you are stuck at square one. Thats a problem if a daily driver.

Indeed a bit of a generalisation - you can't really tell until you take something to bits...

...but generally speaking by the time you start to see a problem then it is often too late (especially if you live in a moist part of the world like me) to save the caliper.

I bought my calipers for my w126 at autozone 4 years ago.

Reman all the way. Let it be someone else's liability. When I had a shop, I refused to rebuild brake calipers due to the liability involved. I also will not sell a used brake caliper or master cylinder for the same reason. Just my 2 cents.