I would say it depends upon what level of services you ARE up to. A kitchen on the event site is usually a luxury--sounds like youve been LUCKY so far. :-) But by and large it behooves most of us in event catering to stay "self-contained" as we never know where we're going to be asked to serve--I've done events in everything from first class on-site kitchens, to a fold up table on a hillside horse stable, (at night in the pouring rain) to a football field on nothing but grass. In fact, I did a wedding once where I had to store all my equipment, and work from a folding table in... a hallway. Welcome to REAL catering.
So first I would assume you are able to prep, cook and sanitize SOMEwhere right? Once your food's made, the main standard for hot or cold transport are thermal boxes, the plastic insulated boxes (like the Cambro brand) designed to hold hotel pans. You can rent them, and they work really well.
The thing about transportting though is in most states you need to be a licensed, "health-permitted" caterer with a permitted kitchen or "commisary" you can legally cook sanitize and work out of.
If you're already set on that score, it's just a matter of pulling together the rest.
I catered for 3 years with either a small van or pick-up truck, and most of the time I dragged at least one BBQ around with me as well. I prepped, cooked and sanitized utinsels from a licensed kitchen, then used Cambros and Icechests for food transport.
And Ive served up to 600 people that way. :-)
ETA: I forgot to answer your extra staffing question:
Depends on whether its a buffet or a sitdown service.
Either way, to experienced caterers 50 people is fairly small,
and if it's a chaffer serve buffet line, 50 is a cakewalk--I can serve that
group in 15 minutes.
So if a chaffer-buffet type gig, one helper should be enough.
If a sit down plated served affair, (esp on china you're providing,)
another one or two staffers, depending on what all you have to do,
site layout etc.