Hi there and congratulations! I used a medela pump very successfully for my son. I think it was called the pump n'style. It had a medium size backpack carrier and a very handy freezer pack for storing the milk-no way do you want to put your milk in the office refrigerator! It was electric, but also worked on batteries-a handy option for travel, especially internationally. (Don't try to use an electric adapter-I nearly caused a fire doing that with my pump.) In terms of your milk flow adn volume, some depends on you, and the time of day, how often baby nurses, etc.
I used to pump alot in the morning, after I fed my son, and then at lunch time, when we were apart, and then less mid afternoon. If you nurse and pump on a pretty regular schedulae you will be ok.
One other bit of advice I would recommend is that you talk with a lactation specialist or someone frmo La lache League-they can give excellent advice adn tips on how to maximize nursing and pumping. Medela sells pumps in stores, but they also have home consultants who can come to you and help you get started-I found someone on their website and she was excellent.
Good luck!
C.