Don't get any less than 18v for a 1/2" impact. I have a Milwaukee and it has been dependable for 4 years of heavy use on a circle track race team. In that time I've had one battery fail. I've also had a Znex 1/2" 18v for 2 years and I think it is a better unit than the Milwaukee and would recommend it over anything out. The Znex has more power and is much lighter. The lighter weight is very much a factor if you use it one-handed in awkward positions. I have a 3/8" 14.4v Znex also and between the two units I can do just about everything on the race car.
http://znex.com/ciw/
http://www.behrents.com/