My first feeling is that I would simply find another place to shoot. Appears to be too many "accidents" for me to think that the range is propery supervised.
Having said that, if you decide to go the body armor route, get the best you can afford. There is no such thing as too much protection.
^ This is my first suggestion. And it would be a lot cheaper than buying a vest.
If you do decide to get a vest, I would go with a IIIA. You can find them starting around 400-500, to well over 1000 if you want to pay that much for it. I would also suggest getting a fitted vest instead of a "one-size-fits-all" measured vest. Everyone's body is different and that needs to be taken into account for a vest to cover what it needs to and still be comfortable.
I would also suggest getting 2 carriers. They will
need to be washed and it sucks needing the vest without a clean and dry carrier (I'm in law enforcement, so occasionally I get called into work unexpectedly and need my vest at the drop of a hat).