As long as you just use the center point, the 5D MK II focuses quickly even in low light. The f/4 lenses in general are not quite as good as the newer f/2.8 lenses, but they are still very good lenses.
I'd say that the 5D MK II is still a worthy camera, and I'd go for the top of the line lenses. However, if you want to use other than the center point, or need to track subjects as they move, then a 5D MK III is the one.
I bought the MK III, and like it, but I have not used the advanced tracking features as much as I should. I did use them in low light, and found tracking was possible most of the time.