The 1Ds2 is a more advanced camera than the 5D1, I have one as a backup and it is a great camera (it was my main for a while till I got a 5D3). The build quality is second to none, as is the AF. IQ is superb, high ISO is good for the age of camera, 1600 is very usable, 3200 less so but there if necessary.
The downside is the LCD screen is poor in comparison with modern stuff and it is a weighty old beast, but that said it has a built in grip and takes massive batteries which last for over 1000 shots a piece. Make sure you get a battery charger (as obvious as it sounds) as a new one is £100+. I use compatible batteries, which I used to get from 7DayShop but I understand are now only selling them through Amazon.