Surely you jest, they've been using aluminum in car cooling systems since the '60s, and the coolant will protect it nicely. I never change old school green that is supposed to change every year, I run it 3-4 years or until I pop a hose and have an excuse to change it. It works even better than the late extended drain 100,000 mile coolant so highly touted nowadays, that stuff is garbage to me. It takes up to 6 months to get max protection, or what the old school gives instantly. I learned the lesson on Focus cars that kept slowly rusting the clear plastic reservoirs, going to old stuff stopped 100% of it. Ford orange or yellow, whatever you want to call it, GM Dexcool, the commonly sold green that 'mixes with all types and brands, and good for 100,000 miles' at parts stores, all that stuff is trash. I'll be crying when they drop the original stuff, I'll bet right now they are already cutting it to mix in more of the commonly used stuff used now. I'll know though by looking at that coolant tank.
Every six months........ ........LOL. I can pull aluminum parts 10 years later and virtually no damage there at all. The outside of the part will be far worse. Not in a hard water area, I have never used distilled water in coolant or batteries either.
The one or two degrees you get from water wetter are worthless for the price you pay. I sure sold plenty of that stuff to people at the parts store though, and thank you for the incentive there.
I've never done an electric fan conversion, but I do know one thing. The commonly available race type electric fans are garbage, they will not last anywhere as long as a good factory fan intended for a vehicle to drive on the street day in and day out. When Detroit first went en masse to electric fans I predicted there were going to be lots of engine fails due to fans dying. Of course that was based on my thinking on electric fans used on race vehicles, which didn't last for spit. Products like Flex-a lite, Hayden, and such. The engine fails didn't happen. I ran through a string of FWD cars, two Tempos, a Contour, two Focus. Get this, not ONE of the factory fans has failed yet, and all cars are still running. The oldest Tempo is an '88 and even though the fan melted at the connector, it still works fine. Now I'm not the only guy on the planet, but I didn't get very many new fan requests at the parts store while I was there either. Detroit stepped up and did its' thing, and those fans are among the most reliable parts on earth to me. So, rule #1 to me would be see if you can find a factory fan and shroud that can work there or be made to work, it should be bulletproof. It might be nice to get a two speed one which is the rage now, single speed but put in a resistor and get two speeds out of it. Running on low they impact your idle less if you are not running a PCM with idle control. Those fans can pull a walloping amount of current at switch on time.