re: A: everyone uses aluminum in cars. copper's theoretical advantage doesn't pan out with soldering and other metals. Aluminum is easy to machine, and cheap(ish).B: 10-15 years ago, it was all custom. now, it's all bolt-together. that's the main reason.
anti-corrosion agents keep stuff from corrodinglubricating agents keep pumps happysurfactants make water "wetter"the loss of cooling effect from using a bit of glycol is far outclassed by the epic surface area of a car / truck (genset?) radiator. bigger IS better.
You will be fine using that radiator with copper - just use automotive coolant - it's got all the corrosion inhibitors you'll need. use a 70/30 water (DI or RO) / straight antifreeze mix, and you'll be A-OK.
I think once you start worrying about the current carrying ability of 16ga wire, what you should really be worried about is the connector.if you're doing this for harddrives, and not something weird, 16ga is incredibly adequate, and nothing to worry about, like, ever.