I. Presume it's safe to use AC condensate in a old engine.
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In theory.

It probably though has picked up 'stuff' from the surfaces it has touched.

If you want pure water I'd spend $2 and buy a bottle.

Instead of chemistry lessons, which may or may to apply since we aren't discussing using 100% distilled water in the cooling system in lieu of mixing it with coolant:

Mercedes-Benz Service Microfiche System, 4/93:

Use water which is clean and not too hard. Drinking water frequently, but not always, satisfies requirements. The content of dissolved substances in the water may be of significance for the occurrence of corrosion. If the water quality is not known, DISTILLED or fully desalinated water should be
used. For water quality see Mercedes-Benz Specifications for Service Products.

Mercedes-Benz Specifications for Service Products, Section 310:

Fresh water quality with a coolant composition according to item (engines for passenger cars and commercial vehicles)

Sum of alkaline earth (water hardness): 0 to 3.6 mmol/l (0 to 20 degrees d) pH value at 20 degrees C: 6.5 to 8.5

Selling its cars worldwide requires some specificity, since water quality varies so greatly around the planet. Follow the recommendations and renew the coolant every other year and your cooling system will be fine.