48: You have research to do. There is an entire book written on this model gun. There is also a very good website hosted by a member of this board, though his interest has waned a bit in recent years.
http://members.tripod.com/bluepike/index.htmI got interested in the Model 21 a few years a ago, did my research, talked to owners, handled several guns, read the book, and eventually found an excellent condition skeet gun that filled the void nicely.
The whole project was rewarding, and I shoot the gun regularly.
The chase is half the fun. It's your money (5 large!), so you owe it to yourself to get as smart as possible. This first 21 you are considering may be the right one for you, but odds are that it's not. There are enough of them out there that you can find the right one if you devote some time to the project.
Good luck, and happy hunting.