- With both the community-maintained packages and the AUR packages, you can already find almost any binary that already exists

- If it doesn't exist, or if you need particular options/architectures, a PKGBUILD is basically a glorified Makefile with standard steps, so you can for example get binaries from a http source and use them directly, or pass exotic compilation flags during the build() phase

- It is of course possible to put dependencies inside a given package

- preparing a package is entirely done by your user, no need for root right. Inside the package you put your files in a fake root, and when the package is done you apply the package to your real fs. By default it is installed under /, so of course you need root rights, but you can install it under any root you desire. This is actually what's done when installing Arch: it basically runs pacman (the package manager) but instead of installing packages in the live environment's root, it installs packages in the mounted filesystem that will become your root after rebooting.