First of all, you do not want a 'hammer drill." You want either a 'roto hammer' or 'demolition hammer.'
The Roto hammer will need to be large enough to take either SDS-Max or Spline bits. (Spline drive is pretty much out of favor). If it's a demolition hammer, it will have take hex shanks, either 3/4 or 1 1/8".
For the money you want to spend, and the relative rare use the tool will see, I suggest you consider the Harbor Freight offerings. Tool snobbery aside, I have found this "Cheap Chinese junk" to perform quite adequately in these particular tools.
If your area is particularly rocky, or you also need to take apart post light bases, I suggest you 'up the ante' to about $900, and get Makita's 35# (half size) jackhammer.
While any of these tools will drive the rod without using a bit, I always use a bit. I see no reason to risk the tool by not having a bit in place. Ground rod driving bits cost about $60.
