You need to modify your fuel system to cope with the extra volume needed.
Yes you will definitely need an aftermarket ECU.
E85 does clean your fuel system, you will need to replace your fuel filter every 1000kms for 10,000kms to make sure you catch all the deposits. Its a good idea to replace your fuel filter at every service after that.
Fuel consumption will increase due to the need to burn more of it. Not sure where you read it will give you better economy.