The cat will be coming by tomorrow morning!

I suspect that if my husband reacts adversely to the cat, he will do so within a few hours, and this means we can return him later tomorrow rather than having to keep him overnight.
The woman who's adopting them out is definitely loaning us a scratching post in addition to a litterbox and food/water bowls, and she will leave us with some cat litter and some food, so we don't have to run out and buy stuff immediately (esp. if we end up having to return the cat). If we keep him, of course, we will buy our own things for him.
Hopefully he will adjust well to his new surroundings, and not leave any "presents"!
pinkyphimu, good point about the litterbox. We have a living room that is currently empty - maybe we'd be better off putting the box there, rather than in a place (like the kitchen) where guests like to congregate.
I do hope my husband doesn't have a bad reaction. To be honest, I'm not holding out high hopes - he's only been on the allergy shots for a few months - but if things don't work out, the woman has agreed to take him back (which is good, because the local animal shelter is not a no-kill shelter

), and we can try again in a year or so, maybe with a younger kitten.