I agree with your reluctance and photographing this sort of scene always makes me nervous.
However, 24-70 lens sounds a good idea. Maybe you can get increased Iso to work without excessive noise or problems with wide apertures and slow shutter speeds. But also look for potential issues between bright highlights and dense shadows. Never easy and it always pays to take a few quick test shots as soon as you arrive.
Flash tends to be my default option, but this also has many drawbacks. In theory, bouncing the flash off a ceiling, possibly a wall, is the established trick, however, high ceilings, dark paintwork, etc can work against you. Also, I have had too many occasions when I have made a mistake with the flash angle so the result has been a directly overhead flash point, or behind the intended subject, which has caused dark faces, sometimes with dark ringed eyes.
That option is suitable for carefully posed shots where you have time to do a test shot and vary your settings but otherwise, be careful.
I tend to treat these situations in much the same way as my wildlife flash shots. Set the camera in manual settings mode to suit the scene as closely as possible then just use flash as a little bit of additional fill flash pointing direcly at the main subject. Vary the auto flash output compensation as required. Once you get the hang of this you should be able to judge each shot in the same way as you would vary normal exposure compensation. Beware of reflective backgrounds such as mirrors, etc.
Using flash and moving around with a large flash unit on top of the camera can make some subjects nervous or cause others to over pose; so either way you lose those nicely relaxed natural shots. I try to quietly lurk around while holding my camera tightly against my body until a situation starts to develop; then quickly raise the camera without any fuss and get the shot.
Whatever you do, this isn't likely to be an easy assignment.
