I've never found the need to marinate Roadside chicken as it comes out plenty flavorful. It couldn't hurt, though. I'd be more inclined to brine before marinating roadside chicken, but that's just me.
As to what cooker to use, the BGE wouldn't be a good choice as you have to mop the chicken every few mins. A weber kettle would be okay, but I'd go with your Santa Maria Pit as it's completely open. My BIL does roadside chicken on his Santa Maria pit all the time, and the skin is always super crispy, which is always a huge plus.