I can say with absolute authority having walked there daily for a decade you will be fine with an 85mm. OK, perhaps 135mm. Seriously, all you have to do is walk up and say left side right side please. If they are belling they sound savage but really don't care about humans although there is sometimes the odd one who doesn't get lucky goes nuts and charges around in mad circles!
Best shot I ever missed was 21 stags swimmign line astern across Pen Ponds, which is where you want to be.
Sadly Peter and Oscar from the coffee stall have moved back to Calabria or I woudl have texted them to give youa shot of Strega in yr coffee! Please post pics, I do miss the place.
PS Be there EARLY!