I see from one of your replies that your father is still living. He may or may not be interested in DNA either, but if they both get tested, you won't have to. Am I right to presume you have an interest in genealogy, just not DNA?
The amount of spit required by Ancestry is fairly small, can be added to over an hour or so if I recall, so shouldn't be a problem.
As to David's point about knowing your objectives, I think you have said it is so that your mother can find 'cousins'. If she means long lost first or second cousins, she may be unlucky, whoever she tests with. If she means 3rd, 4th, 5th etc cousins whom she has never heard of, she is going to be lucky, but somehow I don't think that is what she means.
You can upload Ancestry DNA to other databases, but at familytreedna you won't get a full list of matches with an upload, you would need to test there to get distant matches.
After the first test, wherever she has it done, the cost is over, unless you want a subscription. All uploads are free (but you can't upload to Ancestry or 23andMe from any other site).
Regards Margaret