There will probably be various schools of thought about whether to amend or not. However one compromise solution is to create a raised bed that can be made using an appropriate acidic soil mix for the cammy to grow in. Ie., since these are surface rooters, you could slightly cultivate the native soil and then plop the cammy on top of that sandy soil (which would be good for drainage) and perhaps make a mix of native soil with some top soil, composted soil, and some peat for acidity), then mound around the roots in a bed allowing the top of the root ball to be somewhat exposed, then mulch that. Maintaining the acidity is often an issue for those not having naturally acidic soil but if you have oak trees around, shredding the leaves and adding that around the base would help.

Alternately, you can get a soil test to see what needs to be added to make your soil acidic and buy acidifiers like ammonium sulfate, applying according to the directions. If you intend to fertilize, you would need to check the fertilizer label regarding whether it really contains acidifiers, since products such as Miracid originally didn't (prompting a label change and the offering of a "Pro" line of products now that has an acidifier, leaving the original product essentially a fertilizer-only product for acid lovers). Hollytone is an all-natural fertilizer/acidifier, which is another choice.

The bigger (meaning wider) you make that hole, the better, if you are going to amend it. Be conservative with the organic compounds, you don't want to create a boggy mess! You could use bagged compost, potting medium, even ground bark fines. It is no longer recommended that amendments be added to the backfill when planting hole by hole, for a number of reasons. But if your soil is VERY sandy, and if you are conservative, there should be no problem.

Actually, it is one of those mulch myths about oak leaves. They won't make a soil more acid. You can use any organic mulch that you want. Leaf litter makes a terrific choice. Using a fertilizer with an acid reaction will help and probably be all you need. Hollytone is a great suggestion, because it has a low nitrogen content and some minor elements.

Here is a link that might be useful: camellia planting

Oak leaves contain tannin that during the decaying process will form tannic acid. In fact, just about every part of an oak tree including the acorns, contains tannin (which gives the tree and its leaves the dark brown color) and oaks happen to be acid loving trees themselves. On the other hand, pine needles as an acidifying mulch is what is more the myth. ;-)

Thanks for all the advice. I think the hardest part is going to be digging the planting bed since it's a 15 gallon container and the camillia is about 6 feet tall already!