All I can say to the original poster is"do your research before, not after, you've bought it".
The great thing about the 'Net is that there's info and pics on just about any bike ever made, so read all about it, study (really study) any pics you can find and join any groups or clubs who like them.
The Bantam must be one of the most long-lasting classics ever made. Despite the scorn some gave and still give them, there's loads around, and loads of info too, so no need to rush into buying the first one you see.
The downside of the "classic bike scene" (yukk!) is that there's always some pikey willing to dress up old nails as something worthwhile, but then there's always some ar*ehole who watches Easy Rider and takes a hack-saw to a decent bike, too.
I'd avoid dealers if I was you. Look for the enthusiast selling his long-term bike. Maybe not be catalogue-original but at least someone's lavished some care and attention on it, unlike the Pikey build....