The top tank guns tend to be HVLP= High Volume, Low Pressure. They spray more of the paint onto the work and have much less overspray but they tend to be more expensive and harder to learn to paint with. As you are just painting a bike, a bottom-fed, or Siphon style gun is adequate, much easier to learn on, and genaric models are cheap new (I bought a few at $29.95) will do fine for the first few jobs at least, and then can be used as primer guns. Are you painting your own murals too? I'm just learning about that myself, got a Badger airbrush on sale & a friend gave me a minature compressor, but don't know anybody near here who has such experience so I guess that'll be trial & errrorr. Good luck! 
