I'd choose the 1Ds MkII, even over the 5D MkII, but it all boils down to what you shoot. In fact I did choose the 1Ds MkII, but I rarely go over ISO400, so the worse ISO performance isn't a problem for me.
That said, if you would be satisfied with the 5Dc, the 1Ds MkII is better than it in pretty much all areas.
The issue with going 1-series is that you don't want to go back to "prosumer" cameras anymore...
The problem is, the 1Ds Mark II is going to run about $1500 on ebay give or take. He can get a refurbished 5D Mark II for $1800ish. The IQ of the 1Ds Mark II is not better than the 5D Mark II. I agree it is close in IQ up to ISO 400, but beyond that, the 5D Mark II is better. Same with 1Ds III vs. 5D Mark II. The 1Ds III is much better ISO 50-200, but beyond, it's not better, perhaps just as good but not better.
That aside, if you get a 1Ds Mark II, you'll be fine. But you can get a newer camera.
True and the reason I went this route is the build quality, weather sealing and especially the AF system compared to the 5D MkII. So if those things are important, it makes the 1Ds MkII a good choice.