Back on topic: If I wanted to setup a mobile espresso stand (cart or truck) I'd definitely opt for a propane heated boiler. Generator exhaust fumes are very offensive to a lot of folks, and some communities simply restrict sizes permitted in certain areas. Reliable 240 vac power sources can be few & far between, and are usually allocated on a 1st come, 1st served basis.
The local farmers market I attend has a 2000 watt limit, enough for a small refrigerator and lights. Sound levels cannot exceed 60 decibels, and they do check! All of the markets I've looked into have a nuisance clause; if enough customers or sellers complain about your exhaust fumes, you'll not be welcomed back.