No need to worry about copper poisoning. As already mentioned, copper predates other brew kettle mediums. In fact even today the Pilsner Urquell brewery employs brew kettles, mash mixers, decoction kettles, and lauter tuns which are all built from copper.
While there is no denying that copper can be toxic, your body actually needs copper in smaller amounts. It is only an issue in large amounts. Brewing in copper is not introducing copper in anything close to toxic amounts.
The same can be said of lots of other things. Something as basic as water is also toxic in large enough amounts.