Wow thanks for the replies!
For the two shelters that I've been looking into, they pretty much provide everything (food, litter, bowls, bedding, medical care). So other than transport to pick up the pets, it's basically free, which is rather awesome.
As for length of time, I think I would prefer not to have them for too long, I looked at the application forms - one shelter would like a minimum commitment of 1 month; another gives the option of emergency temporary care which is covering for existing carers affected by unforeseen circumstances, or "short term" which they define as 3 - 6 months. Both of these requirements are fine with me at this stage.
The goodbyes will be tough but I'm thinking we can do it and maybe we should just give it a go and see how we feel after the first one. As some of you have said, I think it will be comforting to know they have found a good home, and also we can look forward to fostering another one after that.