How do I create an installable usb image with all the current updates?
I personally would advise using the debian live scripts. They are included in the ubuntu repositories and there is a graphical font end.
As you can see, it is a graphical front end for live-build. Building a live CD is not always so easy, and all the graphical tools, including remastersys, sometimes fail. Building a live CD, from scratch, is just not an easy task and requires a fair amount of knowledge and experience. IMO learning the live-build scripts is best as they automate the task, but come with a steep learning curve.