Good for you. First, an indelicate question. What age are you?

I've used brush cutters for about 25 years now, starting with a McCulloch and now using a Stihl, which has proven fine, and I am now almost 60. I gradually have found their weight something to bear in mind since serious work (can take several hours) in bad posture and with a heavy machine can be pretty uncomfortable. And I am strong!

I use a cheapish helmet with a visor and ear muffs - it's wonderful. And I dress properly. The little stones can hurt.

I also have several corded strimmers - Flymo, Black and Decker etc., and have just bought a cordless one. Strictly for the little 10 minute jobs.

They come with autofeed, which personally I hate. It wastes cord, goes wrong and you never know whether it's working or not. Maybe I've had bad luck.

Given the size of job you have in mind I would use 4mm cord on a head with one or four strands. I'd steer clear of nylon blades.
As for hitting the gravestones, it's all a question of technique. You must try not to strim the stone all the time because it sure wears out the cord.
all the best
david