What type of wood should I use as an outdoor table top for a BBQ?
I would recommend something that is dimensionally stable, resistant to rot and easy to work, like cherry, chestnut or walnut - for an outdoor food preparation surface, any tight-grained hardwood will work. (See also the "Wood Picker" app)

Fine woodwork that's meant to be kept exposed to the elements should be protected with a marine-grade spar varnish. This requires a bit more effort and care to apply, and will require periodic maintenance - google "brightwork", which is the art of applying finish to wooden structures on boats. There have been entire books written on the topic, but here's an article to get you started.