I'm kind of late to the party, but I might as well contribute.

Housing in glass is fine if temps/humidity can be properly maintained. I keep a BP in a glass tank and have had no problems. Just make sure that your setup is perfected before you get the snake.The care sheet goes more in depth about that....

As for picking out the actual snake, I think that buying one in person is fine, just as long as you know what qualities to look for in a healthy ball python. You want it to be pretty solid looking. If the spine sticks out, the snake's body will have kind of a triangular shape. This means that the snake is underweight. You want it to have clear eyes. Dented eyes are a sign of dehydration. It should be breathing with its mouth closed. If you hear a whistle, see it breathing with its mouth open, or notice that it seems to be "drooling," these are signs that it may have a respiratory infection, which is definetly bad. If the snake seems to be soaking in its water dish a lot, or has little bugs on it and probably in its water, that means that it has mites, which are very treatable but can spread to other reptiles you may have and make the snake miserable. Also remember that the snake should be quarentined from your other reptiles for a few months. This way, the snake cannot spread any diseases to other reptiles. Always handle older herps before your new ones, and wash your hands before and after.

Ok, well I'll stop rambling. Good luck!