6D and 5DIII use exactly the same processor; the Digic 5+, so there is no difference there.
IMHO, the most important difference between the 5D3 and the 6D is the autofocus system. The 5D3 has significantly more, higher quality AF points. How important this is depends a lot on the type(s) photography you like to do.
# and quality of AF points is most important for applications where you're using AI Servo mode to track a moving subject (e.g. sports and wildlife photography). For photographing still or slow moving subjects, the 6D's 11-point system is fine.
There are some other areas where the 5D3 has advantages over the 6D, but AF is definitely the big one. In terms of high-ISO performance, they're virtually identical (I've seen some tests indicating that the 6D actually has a slight advantage in high-ISO). I know one pro wedding photographer who uses 6D bodies by choice over the 5DIII not because they're cheaper, but because they're a bit smaller and lighter, and she doesn't feel she really needs the AF performance of the 5DIII for the type of work she does.
Canon recently announced 5Ds and 5Dr bodies. These are high-resolution (over 50MP) full-frame bodies intended more for landscape and studio work; they're not really designed with high-ISO performance in mind so even without considering the price premium I don't think these bodies are really what you're after.
It's likely that Canon will release at least one more full-frame body before 2015 is out, probably the 5DIV. 6DII is probably also on the horizon, though my personal guess is that any 6DII will come out a few months*after* a 5DIV is released.
Regardless, prices for the 6D have already dropped a fair bit. There are some pretty good deals out there right now so I wouldn't speak against picking one up today if it fits your needs and budget.
As for lens, 24-105/f4 is a very good utility lens to pair with the 6D. Decent range, fairly sharp, not too large or heavy. There are certainly brighter, higher quality lenses available, but they're much heavier, usually have a more limited zoom range, and cost much more.