Hard to tell if they're in decent condition until you actually see them in person.
A lot of the larger tanks I've seen for sale are slightly abused. The people who are selling them store them in the basement/garage for years and then eventually try to get rid of them. While there are some legitimate sellers out there, you always have to be wary of the people who sell tanks that should be TRASHED.
Here's an example of a 100+ gallon tank someone sold last month. See anything that might concern you in this photo?
Anyway, a 40G would be plenty of room, dividing it might require a bit of work, but 16" wouldn't bother a HEALTHY betta whatsoever.
As long as their water is relatively clean, free of other invading fish, & they get food at regular intervals, Bettas don't seem to care.