old nickle metal hydride type they dont hold charge well and will likely drive you nuts
With constant heavy use, you might only get a few years out of a battery. But I think in the US, genuine batteries are only about $100. And there are also a lot of cheaper 3rd party batteries out there as well. I can typically get around 1500 shots per battery with mine - when they're working properly, they're usually pretty good. (But the battery charge indicator is useless. You need to keep a mental tally of shots taken or just recharge it regularly). Also the battery charger has a "refresh" feature that helps keep the battery in good condition.
BTW - Make sure you get the battery charger as they are expensive
Bear in mind the 1Ds Mk ii is a much bigger and heavier camera. You notice the weight when carrying it all day. It also draws a bit of attention.
I've never used a 5D Classic, to compare, but I love the image quality of the 1Ds Mk ii at 100 ISO. Noise becomes apparent reasonably quickly and is obvious at 400 ISO and over. But it still captures nice images all the way through its range. I've read some articles that suggest that it is less sharp than a 5Dii. Personally, I don't think that's the case.
On the plus side, it is fast and responsive. Apart from the size and weight, it is a pleasure to use. The downside?.....it lacks video. Also, you need firewire to connect directly to the camera for tethered shooting etc and later versions of Canon DPP don't seem to support this.