That's why I still have my a/c hardware. It doesn't work, but long term I will get it back running. I love a/c.
I got the 12" fan. It's wired to the stock circuit - no extra relays. It's cheap for sure, mounting hardware is basically some fancy zip ties. haha. It is kind of slick though, just slide them through the rad's fins into a fitting on the other side.
For me it's not possible to get another fan in there as my downpipe does not provide enough room. You can see the space in the pics.
100% is fine I think, biggest thing is extra wear on the fan motor. But then again, if you can utilize the stock system, why not. It might simply require a slightly higher amp fuse to run on stock wiring.
If you were still concerned about overloading, you could certainly install a relay off of the existing circuit. But I think thats overkill really. The wires are sufficient to run 2 fans.