Has anyone ever had a client ask for brushing their dogs' (4 of them) teeth as well as giving medication (on top of food and water) and cleaning a cat's litter box before?

First off, you should never feel sorry that a client has to pay a certain amount because it seems a lot. They own all those animals and should be prepared to pay whatever it costs to have them cared for in their absence. Some sitters may offer a slightly reduced rate for additional animals but that is up to you. I would never arbitrarily give a discount when there's so much work involved.

As for brushing the dogs' teeth. NO. No Way. Not for the amount they're paying you per visit. Moreover, most dogs actively discourage anybody trying to brush their teeth. I doubt it would be dangerous for you, but it certainly would take far longer than a drop-in visit with all the other chores you've been given. Suggest that the client allow them to have a dental stick/chew instead, while they are gone.

Only you can determine if a job will work out for you. There are many demands being placed on you by this client. Be honest and upfront about what you feel you can do and can't do. If the enclosure seems unsafe, then tell them because you will be the one who is held responsible.

Good luck!