+1 on iron legs. I would go with black square tubing.
Issue with slabs is movement/cracking/cupping/warping/twisting -- EVEN WITH FINISH ON ALL SIDES. Nuff said, be careful how you attach legs allow LOTS of room for movement. A slab this wide could fluctuate as much as 1" seasonally if left outside any restriction will cause cupping.
Whatever you do, don't allow direct sunlight it may turn out to be a twisted mess.
No expert on finishes, but I like the epoxy idea.
No matter what you do, there may be issues keeping this slab outdoors.