Go for a 5dc or 5d2 if your money isnt enough and buy the 24-70. Those 2 ff are still a lot better than t4i and you'll be exposed to the DOF of an FF. This way, you won't think of upgrades until at least your camera goes down. As for me, if i have that kind of money, it will be a 6D + 50mm f1.4 + 85mm F1.8. Ive got already a 28mm to cover the wide end. Agreed, primes are more inconvenient to use but I prefer the IQ of a prime. But if youre earning money through photography, a 24-70 is a must.
I'm afraid I'll have to disagree with you. Gone are the days that prime lenses are better than zooms. As I said before, I already purchased the 6D + 85mm f1.8 and promptly returned them because the 85mm had unimpressive sharpness, color, contrast, and aberration correction compared to the 70-200mm f2.8 IS USM II that I sold. I was willing to give up the focal range versatility, but the 85mm was simply inferior in all other aspects as well. There's nothing really wrong with the 85mm f1.8, but its 20-year-old design just doesn't stand up to a top of the line zoom that costs $1800 more, which was what I was used to using before.
Also, I'm not quite sure a 5Dc is better than a T4i. The sensor is, for sure. But what about the ergonomics and controls? Autofocus? Video? (I do shoot video occasionally, mostly at home for fun.) Actually, it would appear that the T4i is better than the 5Dc in every way EXCEPT sensor. Not a very convincing argument for the full frame crowd.
I don't know if you've got a bad copy of 85mm but I've tried it and found it's acceptable for its price. True, it might not be able to match 70-200 open wide but stopped down, I think it's almost comparable especially if you take into consideration its price.http://www.dxomark.com/index.php/Lenses/Compare-Camera-Lenses/Compare-lenses/%28lens1%29/322/%28brand%29/Sigma/%28camera1%29/0/%28lens2%29/241/%28brand2%29/Canon/%28camera2%29/0/%28lens3%29/408/%28brand3%29/Canon/%28camera3%29/0
I think you're into more of a consumer than an enthusiast/professional at least for now since you're also looking at video autofocus (even 6D and 5D3 doesn't have one). By all means, T4I is better than 5DC on that area. As for ergonomics, it's subjective from person to person. I have suggested a good 5DC or 5D2 camera because I know a lot of photographers who want to go FF but didn't go FF all at once just because of the price (I'm one of them). While it's still better to invest on a lens than a body, you also should know that you can only take advantage of the 24-70 or 70-200 fully if you're using an FF body. ISO and DOF aside, I for one value the fact that I can go for a wider focal length using a 24-70 and an FF than using it with a crop body. That alone for me makes investing on a 24-70mm lens more worthwhile. Of course that maybe isn't the case for you. If you think you can be happy with the T4I and 24-70, then by all means, buy it. You can still upgrade later if needed. I'm not here to convince you otherwise. I'm just letting you know what I feel when choosing between an APS-C and an FF body.