You will need a "siding wedge" or "pillow" to translate the irregular surface of the siding into a flat surface for mounting the camera. If I remember correctly there are links to them in the
Wiki, available at Home Despot for one. You've probably seen them used for coach lights next to doors, for example. A box isn't necessarily needed IF there is enough room behind the mounting surface for the RJ45 connectors. The "pillow" can give you enough room, probably. You will need a 3/4" hole to accommodate the RJ45 connector unless you terminate the CAT after getting it routed.
If you're mounting to a soffit you can get away without any holes in the house by mounting the camera to a strip of 1-1/2 or 2" aluminum flat stock from the flashing, where the soffit tucks in, to the house where, again, the soffit tucks in. Getting a piece of soffit out can be a little tough, but it can be done fairly easily and that allows routing wiring without any holes.
In terms of mounting location and position, you're probably better off experimenting with temporary mounting to get the exact angle you want and the WAF. If you play with positioning for soffit mounting that flat stock trick works pretty well and can be moved very easily.