In theory it's ok, but air conditioners tend to leak oily goo and rust when they get old. Look at the surface of the water to see if it looks oily. You're not supposed to drink the water, because bacteria and mold can grow around the coils, but that's not as big of a deal for a plant.

Unless the A/C has failed, there will be no oil on the condensate.

I can't think of an A/C unit that has steel coils that would produce rust......coil are aluminum fins on either alum or copper pipe.

Use the water, it's how distilled water is produced.

I have a unit right now that leaks a little coolant on a hot day when it is running a lot. It bubbles out and looks like oil. The unit is fine until it hits about 100 degrees outside. And all of my old window units are rusty. It's mostly the metal housing that goes around the back. The water sits inside and rusts everything.

I do happen to have a hosta outside right underneath the window unit's drip, and it is doing great. In theory, anyway, rust in the water is merely supplemental iron, and a little is no problem.

You need to replace your window units....what you are doing is not very green.