I agree with Marcos. Interphoto is a good lab. I used them for E-6 quite often, and I'm sure they will do a good work with your B&W film. According to the information on their website, if they receive B&W film before 10 am, it will be ready the following day.
Another good lab, although more expensive than Interphoto is
http://lab35.es/. It's not far from the Gran Vía/Puerta del Sol area (20 minutes walking or 10 minutes by underground/metro).
I suppose there are more labs in Madrid, but I can recommend Interphoto and Lab35.
I would be surprised if 50 rolls of film are over the limit for customs at the airport. Anyway, you can buy film in Madrid at reasonable prices. In my experience, the place with the best offer of fresh film in 35mm, 120 and LF is
http://www.fotocasion.es/ . It is located in El Rastro, exactly the place where you will probably be on Sunday morning (Yes, they open Sundays). As far as I remember, Fotocasión offers a film processing service, but I'm sure they don't do it themselves.
I hope you enjoy your trip.