I'd get a 1 series Camera like the 1D X, or at least a 5D MK III. Canon refurbs are excellent, many of us who have bought them find them to be even a little better on average than a new one. They get more and better QA than a new camera or lens.
The Refurbs now have a 1 year warranty as well.
Also note the Canon Loyalty program where you can trade in a old broken Canon film SLR or powershot for 10-15%, maybe more off a refurb 5D MK III. http://shop.usa.canon.com/shop/en/catalog/cameras/refurbished-eos-digital-slr-cameras/eos-1d-x-body-refurbished
Nikon jpegs tend to have the saturation turned up while Canon tends to have them with less saturation. You can adjust it anyway you like, its merely a difference in philosophy about the level of saturation.
Nikon lenses are just as much noted for inconsistent AF as Canon, that's why they offer "Fine tuning" in the better Nikon bodies. Its nothing to worry about, but the facility is there if you want the best possible autofocus accuracy. Normally, you will not see much difference until you get to f/1.4. or f/1.8.
Normally, the expensive lenses like the 300mm f/2.8 are going to be right on AF wise, but there are certainly some that need fine tuning.
I'd recommend selling the older non IS version if needed to pay for the 1D X. I think your friend would have agreed with that. There is a table of lens selling prices in the price watch section, I'm not sure if the non-is version is there, its pretty old, and Canon no longer services it, but you can usually find a third party service company to do a limited amount of work on one. I'd sell it while its still in near mint condition.