Are you building for yourself or professionally? I have had to step back for a few years to support my son, but now he is old enough to jump in the mix so I am in planning/saving/scheming part of getting back into it.
If he really wants to wow people while cruising...I'm just helping out a friend, not doing this professionally (assuming that Q was directed at me).
As far as the headphones, I believe that is illegal in Virginia. On top of that I wouldn't personally recommend it - headphones reduce situational awareness more then a loud stereo IMHO. Plus, yes, I imagine my friend wants this for boulevard cruising too.
I am a firm believer in having air-horns on bikes. Even my Vespa has one.what about under the panel/cover just in front of the saddle bags. There is a descent amount of dead space there. I am looking for train/truck air horn set for the bike now. I had one of those caravan mother experiences and went sliding all over when my back locked up avoiding her. I tried baltting the exhaust and realized she couldn't hear them over the cell phone and kids in the back of the van. I also disvoered the bike has a rev limiter on it. There may have been an agressive downshift and accelration in there once things got back under control. My son was on the bike with me. I was seeing a little red. I would have loved to have had some train horns and scared her *** off into the median. At least she is wrapped in steel and glass.
For a horn you shouldn't have to. The Harley airhorn (which isn't the loudest, I admit) will fit right where the stock horn is. The Rivco horn (which is much louder) can bolt onto your engine guards or frame rails. It is pretty so you don't have to hide it. I'm sure if you are clever you could find somewhere to mount a Stebel Nautalis, which is the loudest I've heard shy of train / 16-wheeler horns.I know. I am considering giving up a little bag space somewhere for this. I love loud pipes but I will try to stay out of them for now for the sake of the neighbors.