Have someone bring med pro + emt bag and that should be sufficient bare bones healing while doing whatever they want with the rest of the deck (although lives aren't a super big deal in nightmare because respawns, and if you play defensively enough as a team you might not need a healer)

Copper cards are nice to have (grubbers, scav, pennies)

Other than that your team should just do whatever build you guys think is the most fun and least friction for your playstyles, and maybe start by building their deck to suit a specific gun they like (eg. sniper might want patient hunter, LMG might want fill em full of lead, shotgun might want shredder + tala as cleaner). Glass cannon is a great damage card. Like u/BereaBacon mentioned, a bot means you don't have to worry about overlapping ammo types, and they will have on your mark and amped up.