At home, I normally use a kettle barbecue with lumpwood charcoal. I know how to allow enough time, and I have backup microwaves, ovens, and grills indoors for pre-cooking and coping with larger numbers.
I seem to have been volunteered to organise some off-site barbecues without much in the way of backup facilities other than a microwave and electric kettle. I'm tempted to get myself a reasonably large gas barbecue (but small enough to lift into my Suzuki Carry van), because I will still use it at home. I already have the propane cylinder and regulator for the big gas torch (and a spare butane one).
I realise that this is one of those "piece of string" or "how much do you want to spend" questions, but I'd still be interested in the collected wisdom. Could be looking to cook for a dozen people, perhaps more. A South African of my acquaintance is a big fan of Cadac systems but they look a bit small to me for the possible numbers.