Congratulations Lady - well done on being a foster!
It's very unusual for a foster carer to need pet insurance - normally foster dogs' bills are covered by the rescue who is responsible for rehoming them, and their public liability insurance should also cover any 3rd party type incidents. Before you spend any money, check this with the rescue.
Do you have an existing dog or any other animals that the dog would need to get along with?
Most rescues will do what is effectively a "homecheck" for new foster carers - so fences and garden security checked, and a discussion about how vets bills/visits are to be managed, reporting arrangements, support that is offered to you etc.
See if you can find out what food the dog has been on (if known) so you can continue with that for at least a few days.
Is the dog coming from a relinquishing owner, a pound, a kennel or directly from abroad? It's useful to know things like whether it's housetrained, any previous experience of living in a house, and anything known about its nature. Even if the dog is housetrained, you should be prepared for accidents in the house over the first few days. Marking wees are normal and upset tums are too as the moving/stress can affect digestion. Make sure you have good wee cleaner stocked up. 
Check what ID tags the dog will be wearing and if is already chipped. Make sure you have a note of chip number, and who to contact if dog is lost etc.
Remember that the first few days are when the dog is most likely to bolt, run or escape and when it is most vulnerable since it does not know you or the area. Make sure therefore that you and all family members are fully briefed on keeping doors/gates shut, that dog is wearing well fitting, breed specific collar and sturdy lead, you don't let dog off lead, and that you are super careful in gardens, open spaces etc.
If a bitch, you may or may not know spay status. It's very typical for season to start after moving house - seems to trigger it v often. Rescue should be able to advise you on how to manage/proceed with this.
Take things v gently for the first few days - concentrate on building a routine for the dog and don't over-fuss (no matter how tempting!). As dog relaxes and becomes more confident, you can gradually interact more.
Good luck!