I've done a few using nothing but my 110v 20g I bought at walmart for $80 coupled with the cheap paint gun. Cheapy guns aren't HVLP and don't take much air since they use higher pressure to get the job done. But you can find HVLP's that only need 5-7cfm to work right. But you'll be spending more than $20 for them. For the most part you can shoot for quite a while before the tank drops below 50 psi, then take a 2 minute break and start up again.
I've never used a gravity feed, been using the same style since I redid my first car back in highschool
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=43760 Like this, pulls 1.5 to 6 cfm. Pretty much any compressor can keep up with them.