Any power tool is dangerous
You can cut slabs with a small grinder- probably safer for a first timer than a big one, so long as you take care
What do you need to do? Lift a slab, dig a hole, fit the post and cut the same slab to fit around?
What helps is to have a bag of sand to go under a slab, and don't try to cut all the way through. Cut your line and make it from A to B then go deeper. The slab normally breaks easily