How hard is it to assemble the HAH?

As with most tasks, it really depends on where you are starting from & the skills that you possess. If you can't follow detailed instructions, you will need the help of somebody who can. You will need a PC, loaded with an FTP server, to break into the Livebox for reflashing with the HAH firmware (or buy a pre-reflashed unit). You will require basic soldering skills and some time to solder up the PCB.