This is tricky. I went and bought a Wagner, (haven't used it yet), but word in the forums is that it's really loud. The client will be working at home during the day, so I can't have that.
Plus, by the time you get the handset plugged into the wall and use a siphon, I don't see a big portability advantage over a floormounted unit.
I should clarify; my experience with airless was with an ancient Graco beast . . . beautiful machine, you could spray about as fast as you could climb the ladder, but it was very limited in terms of pressure. Anything less than 75% and you'd start to get fingering.
Also, I'd like to start finishing guitars on the side, so it may be worth really looking into a fine-finishing/airless combo. Our local HD doesn't stock the Milwaukees, any suggestions?