I have mine mounted just behind the battery next to the rear fender so the 18" kit worked just fine. That's on a Standard so you'll need to evaluate if it would work for your Touring. It's really simple to wire up.
For the GPS, I would recommend that you connect to one of the unswitched ports. This will allow you to mess with the GPS without having to turn the key on. The downside is that you need to be mindful of turning the GPS off when appropriate.
The speakers/amp can connect to the switched ports.
When connecting the battery leads to the PC-8, do a close visual inspection to make sure that the washers have not hung up on any other parts thus compromising the connection. (learned the hard way)
The EB stuff is first rate quality. I would also consider doing the headlight mod while you have everything unbuttoned.
The taillight circuit is nearby under the right side panel and you can use it to trigger the switched lead.