I agree with the temptation £500 cheaper for the one im looking at, but the 1Ds III is about the same performance noise and IQ wise as a 5DMKII?
I don't have any first-hand experience comparing those two bodies, but considering the 5DII and 1DsIII share the same sensor, I'd imagine that noise and overall IQ would be pretty darn close between them. I believe brainsquibb has experience with both the 1DsIII and 5DIII, so he can probably offer some better insight.
I would say the only reason I don't get as many shots as I should is by trying to keep the ISO in the lower end of the scale and risking a bit of blur rather than being confident in ISO performance that I can go up higher and not worry.
Been there, done that, and as an old film fart, I can certainly relate
That said, although I felt I was always pushing the ISO limit of my 5DC, I never found it lacking in that department. The 1DsIII would be far better. As you can relate, car interiors can be like a cave, and this is usually when I push the ISO the hardest. I know many pro car photogs that are far better than I am that have been shooting with the 1DsIII since it was brand new. I don't think you'll feel that it's lacking in terms of ISO or overall IQ.
At the moment it is the 5D MKIII because of its size and relative usability to be an all round monster. But I always wanted a 1Ds and im sure we've all been there!!
How about purchasing a 5DIII, giving it a go, and if you don't like it, you can always send it back. In the interim, you could always purchase something along the lines of a 1DII just to get the feel for a 1-series body, then sell it without incurring much or any loss of money at all. Of course, if you get spoiled by the 1-series, then it might be a sign to go with a 1DsIII