I had to replace the battery on my 98 ML320 about a year ago. One other problem I had which you may keep an eye on is the harmonic balancer. It went out last July. They supposedly had some problems with them on the early models.
The failure was that the rubber between the inner and outer metal portions deteriates. I found it before it came apart. When they do they can do quite a bit of damage.

Wayne
82 300SD 200k
98 ML320 78k