I'd visit it at different times of the day and see how the light falls. It's ok to have direct light on people, but you don't want one person to cast a shadow on the other person's face. Each person should be similarly lit.
Some reflector is good as long as it doesn't make people squint. If it's shady you can also use a flash, but how bright you can go with the flash is mostly determined by your sync speed. A slow flash sync shutter speed means a small aperture, which means your flash won't have the power.
I'd think an 80mm would be fine for a group photo. And have the camera up high enough so people at the back don't fall backwards. Too long a focal length will not provide good DOF for a group photo.
I think TMY2 film is fine for anything upto 16x20 with MF for unobjectional grain. A compensating developer will tame bright day contrast. I use PMK. If it's overcast a normal developer might be better. You could use the tmax100 or delta 100 for more grainlessness, but I haven't much experience with it.
Finish off a test roll and then use another one for the actual shoot. Then you can develop the test shots first and see if you need any developing changes to accommodate the scene.