WSUS Offline updater is pretty good. It's a bit clunky as a first time user but once you get used to it it works well and can save time if you do updates often enough. All the program does is provide a GUI to download the updates direct from Microsoft then apply them in the order windows update does. It requires several reboots just like downloading the updates via the built in windows update application.
Basic directions, download, extract, run the update generator in the main folder as admin, pick what you want it to get updates for and download. The under the client folder on the machine you want to update run the update installer and follow directions. You can even make ISOs of the updates if you want.
Adding the updates to the ISO is something you can do as mentioned, but that's harder to setup for the first time. As for downloading a pre updated ISO from the darker corners of the web...I would consider it for one of my PCs but wouldn't on a customers for fear of potential questionable software put in by those creating them.