It would be a modified I4 engine to be able to handle the higher ethanol content. Backed by GM.
Here is an article on E85.
http://www.cars.com/go/advice/Story.jsp?section=fuel&subject=fuelAlt&story=e85
How many ethanol stations do you have around you? Check
http://neare85.com/ I only have one and it is in Salton City, CA over 60 miles away.
Quick summary.
"......
PROS
+ Renewable fuel
+ Uses up to 85 percent less imported petroleum
+ Less money goes overseas
+ You don't have to pay a premium for a flexible-fuel model
CONS
- Gets fewer miles per gallon than regular gas
- Environmental benefits unclear
- Possible effects on global food supply
.....
E85 - 85% Ethanol and 15% Gasoline"
As for your original question.
Yes modified, regular gas vehicles can also be converted, cost extra though.
Is it preferable? Not to me, if you live in a large metro area with E85 and it cost much less than regular gas, than it might be to you.
The problem that comes into play with Flexfuel vehicles is fewer MPG and the additional initial cost of being a FlexFuel vehicle. When does it even out where you are actually saving money when compared to using regular fuel? Someone here has posted some calculations to answer that question.