You would save on the cost per litre, but the engine would use more fuel per mile, compared to petrol, so there is no real cost saving.
AFAIK, there is no saving in road tax. My SAAB runs on E85 and i pay the normal rate of road tax.
Conversions to alternative fuels can be dodgey - remember LPG? A lot of people converted patrol cars to LPG and found the performance and reliability suffered.
Also, whereas Maxol sell E85 in a handfull of filling stations, there is no guarantee that they will continue to do so. In fact, Maxol have indicated that if there is no concession to E85 in the upcoming budget, they may have to discontinue selling it.
Personally, I wouldn't convert a 9-3.