Rescue has asked me to foster (UK), what questions should I ask them?

I have two elderly dogs, one of which came from this rescue 10 months ago. They have asked if I would consider a long term foster for another old dog. I’m very tempted but haven’t fostered before. They are calling me later today to discuss in more detail – what do I need to know/consider to made this decision?

(Deliberately keeping things vague so as not to influence responses but happy to answer questions.)