What prevents you from trying to stitch with your 5DII to see whether it gets you where you want to be? It would only take a few hours to figure this out by yourself.
I would suggest the following course of action:
1. Select a typical scene you work on,
2. Identify the longest possible focal length that enables you to get the people in the scene in one single image captured in portrait orientation,
3. For a start, just put the camera on top of a level tripod, start by taking the image of the people until you get it exactly right,
4. Mark down the angular position of the tripod head,
5. Then rotate the head left so that you see a 30% overlap between the first image and the second one (your models are still into place),
6. Add a few more shots to the left,
7. Go back to the initial tripod head position you marked in step 4,
8. Then rotate the head right so that you see a 30% overlap between the first image and this one (your models are still into place),
9. Add a few more shots to the right
This should have take no more than 2 minutes since the end of step 3 (more like 45 seconds if you know what you are doing).
10. Download a free trial from Autopano pro and/or PTgui Pro,
11. [Example of Autopano Giga] Import the images with "select images" and click on "Detect",
12. In the right part of the interface, click on "Edit",
13. Change the projection mode to "planar projection" (4th icon from the left) if it is something else,
14. Click on "render" in that window (3rd icon from the right)
15. 2 or 3 minutes later you will be able to open the image in Photoshop,
16. Try a print at the largest possible size you think you are likely to need -> this will tell you how far you current camera can take you in terms of size when using stitching
17. Multiply this size by 1.2 to figure out how far the 5DIII or a D800 would be able to take you.
You might see some issues, they will typically result from the lack of usage of a pano head and the lack of masking in the super basic workflow I explained above.
Those would need to be checked with a pano head like this one: http://reallyrightstuff.com/ProductDesc.aspx?code=Ult-Pro-OPP&type=3&eq=&desc=Ultimate-Pro-Omni-Pivot-Package&key=it
And of course some more learning of pano software. :-)
This may or may not enable you to save a lot of money in gear. Only you can figure out whether it makes sense it terms of workflow and the time and skills this requires for your applications.