Just throw my satin spraying experiences in the mix...
I used 4:1 as a Paint:Thinners ratio, and 40psi. I opened the garage door, and that was it as far as ventilation goes. Using mineral spirits meant it wasn't too smelly - so I actually got NO complaints from my wife!! Try that with a Urethane...
And the same comments apply re runs - if you get any, just quickly wipe them (and the roller idea sounds better than my finger) and let the coat self level. You can rub them out when the coat's dry anyway...

It depends on the size of your car and the size of your garage, but I'd think that a 20 foot hose might not be long enough, and you don't want to drag the hose across a freshly painted panel...

One thing I did find when I was using a touch up gun for door jambs etc was that the paint would dry at the tip of the gun, and therefore the paint supply disappeared. The cure was to periodically wipe the tip. That only happened on the touch-up gun however - probably because the nozzle was SO much smaller than the 1.2 I used with the real gun.