Well, I'm just going to take it from the top and you guys can decide which way to go. The game changer when it comes to power is the espresso machine. It's a power hog. So, you'll have to decide if its something you want to carry. I feel comfortable saying that everything else can be ran off of marine battery/inverters. But as soon as you add in the espresso, you'll have to either go to a generator or propane heated espresso machine. If it helps with your decision, last month espresso drinks made up 2/3 of my beverages sold and the other nice part is that you're adding milk, which is marked up as well. My average ticket price for an espresso drink is $5, my average ticket price for a coffee is $2.25.
Powering the espresso machine can be done by propane or generator and you will have to search for an espresso machine that is specific to those power sources or you will have to convert it yourself. If you don't have experience, I would suggest just getting one made for those needs.
Generators can create a lot of power. Much more than propane. They allow you to hook other appliances into the generator. They are more fuel efficient than propane and generally the prices for gas are a bit cheaper.Cons:
They are also noisy as hell and stinky. The exhaust has to be pointed away from your establishment. They are heavy to lug around. They require maintenance.
Quite. Easy to lug around. Odorless.Cons:
Carbon Monoxide threat. Less power, which means longer recovery time (recovery time is the length of time it requires for the water in your espresso to heat up again before you can pull another shot).
Personally, if I were to do it over again, I'd probably go propane. Right now I have a electric espresso machine, which is awesome at how quickly it recovers. It never leaves me waiting, however, carrying that generator around is a pain and I'm thinking about franchising, so I just don't see an employee doing it. But your situation might be different. The downside to propane is you will most likely lose sales due to a long line. But I'm not sure that's a bad thing, because long lines are good advertisement and you're going to have to lose a lot of sales before you can counter balance the cost of generator plus equipment.