I'm no group portrait expert but I think you've put yourself in a difficult situation. If the location is fixed, I would definitely look into fill flash. Assuming you don't have a softbox or umbrella (and don't have a powerful enough flash to use them), I'd consider matte acetate over a flash like a Vivitar 283.

I'd go with the 80mm lens. It will allow you to get closer and therefore get your flash closer without having to rely on stands, cords, or electronic triggers. Plus greater depth of field.

I'd use ISO 400 film as grain is not likely an issue and will allow smaller aperture. Are you doing the film processing? Hopefully not as it adds another variable that you (rather than someone else) needs to control. I don't think it matters much whether Kodak or Ilford unless your lab has a particular preference.

For me, it's hard to get a group photo with that many people where it doesn't look somewhat "wooden". Especially in a business environment. One solution, if it's feasible given the intended use of the photo, is to have multiple photos with smaller groups. Say 3 photos each with 3-4 people having a conversation. Staged? Yes, but certainly no more staged than any other group photo. Or find some other way to have the group interacting.

If it was me, I'm not sure I'd use medium format unless the quality provided by the larger negative was required. While the TLR does give you the advantage of being able to see the group at the precise moment of exposure, you are limited to 12 exp. before reloading. Also, you want to get your camera up high, which is more difficult to do unless you have a prism.

In fact, at the risk of angering the moderator gods, this may be a time to consider a non-analog medium, tether to a laptop, and get immediate confirmation (given that you're new to MF and B&W). You don't want to call them back out the next day.

I would use an incident meter if possible. Or go out the day before with a grey card then adjust for differences in outside light. I'd bracket 1 to 1.5 stops in both directions. Don't forget a good lens hood or other mechanism to prevent flare.

Give some prior thought to how people will be positioned based on their height, business relationship, or whatever. See if there is some way to loosen them up a bit (depending on the group dynamics). Have an assistant that will help make sure that the group is positioned properly and look for blinking, squinting etc at the time of exposure.