The plastic that the float is made out of is just fine. Ever taken a sending unit out of a new car? It's mostly plastic. I ran my junky aftermarket sending unit with the plastic float on it for over 10 years and the float still looks perfect. I just removed that float and put it on an OEM sender yesterday.
The main problem with alcohol is eating rubber seals (such as the ones around accelerator pumps) and old rubber fuel lines.