Dubya,
I'm a pro and often use a 5D3. They're reliable workhorses and the prices have dropped considerably since the announcement of the 5D R and S models (which you can only pre order at the moment). I prefer the the 5D3 to the 7D2, but the 7D2 would be a big step up from your current camera. I think the 5D3 would do anything you want to do and more.
A buddy of mine is testing the 5D R for Canon now. He reports awesome resolution though slightly less dynamic range than the 5D3. The files are huge and you'd better have really big hard drives and cards and a very fast computer. I suspect the amount of data would be way more than you need and it might require major upgrades to your computer kit. You also have to process the images with the Canon software currently since Adobe softwares don't yet support the new cameras' files. That's a drag for now though that will change eventually.
I'd recommend you borrow your nephew's 5D3 and shoot it, if you can, and see how you like it. If that isn't an option, rent a 5D3 and/or 7D2 and try them out before you buy one. Some rental places will apply your rental towards the purchase price. That's a good deal as it removes any doubt about what you want.
