I'd ask about their pedigrees aswell - what is their prefix, does she have their pedigree certificates ?
All reputable breeders tell owners that they will take back cats they have bred if it is ever necessary, and I would expect someone genuine to at least have informed the breeder what was going on. If I had to rehome my cats, there is no way I would charge - finding the right home would be far more important.
Check on the booster situation - is it just the annual booster they need ? If not, does that mean that they haven't been vaccinated regularly in the past ?
Can you get permission to talk to their vet, or get them vet checked before you agree to take them ?
Also ask lots of questions about toilet habits - number and type of litter trays, type of litter, history of illnesses / accidents, type of food they eat, do they go outside ? If they are indoor cats, they will not be streetwise and you would be wise to keep them in. If they are outdoor cats and you want to keep them in, there is a risk they won't be happy.
Try to make sure that the transfer of ownership pedigree certificate is sent off by the owner, to officially put the cats in your name - not vital if you are not into showing, but useful.
Tonks are gorgeous though - loyal, clever, affectionate and vocal like Siamese - I really hope it works out