Ethanol has been in use in Brazil since the 70s. The fuel is cheaper than gasoline, but you need to burn more of it. The rule of thumb was that the break even point was around 70% the price of gasoline (but that was applicable to the mostly compact car fleet of Brazil - every vehicle would have its own number).

It definitely pollutes significantly less. You also have zero issues with carbonization in the motor as alcohol has a decent detergent action. You should get a bit more life out of your catalytic converter.

You get a few "free" HP if the conversion is done right.

Cold mornings are your enemy. Alcohol takes longer to heat up your engine, so there's a gasoline reservoir for cold starts that the on-board computer doses until the engine is warm enough to not sputter out.

If your conversion leaves you with a flex motor (any mixture of gas and ethanol), you can switch to E25 in high winter (or eyeball it at the pump for something like 50/50). Helps avoid wasting fuel heating the engine from a cold start when it's white out.

Ethanol is highly hygroscopic, so components in your gas train that don't deal well with water can start to rust. This was an issue mostly in the days of leaded gas, but nowadays all gas has some ethanol in it, so you're probably fine.

Correct, don't use premium fuel unless your engine requires it.