I was talking to my step son the other day, a brick layer. The breakfast burritos were their favorites on their job sites. He told me to just make them big. 

In regards to coffee, just get a pour over perk, probably two, from WalMart or Sam’s and get up a half hour early to start, and buy a few inexpensive air pots. That is how I ran my first mobile coffee unit years ago, and it worked. I used Maxwell House, cheap at Sam’s and tastes good to most in the am.

And allot of bottled water, the big bottles. I doubt you will sell much soda pop, but a nice brewed sweet tea would go over very good in my book. Lipton makes some cold brew tea that is awesome, no electricity required. You just need some coolers with bags of ice, the guys will not care as long as everything is clean and sanitized every day. And let them know that, because it will matter. Keep a bucket of bleach water around and use it. 

Just my .02 as a guy that worked in the oil patch and also served the food.

Mike

And on edit: You will find that most guys in the field will carry their own mugs or insulated containers for their drinks, so keep that in mind in regards to what you charge them or the company. Some of those mugs can be up to a gallon!