1) A Pump - If you will need to pump at work with a limited time frame, you will need a double electric pump. If you will only need to use it every once in awhile then you could probably get away with a single electric or a hand pump.
I used the Medela Pump in Style (PIS). It worked very well for me. It is a little pricey but from my research I learned that you pay for what you get. I also have only heard GREAT things about Medela pumps. From customer service to product reviews, it has always been good.
2) Storage Bottles - Now, that you have a way to get the milk out, you need something to put it in. Most pumps come with a few storage bottles. I got these storage bottles from BRU. They were ok. I wasn't crazy about them because the lids were sealing disks and rings. So unless you got them on just right, they could leak if they tipped over. Plus, the oz. markings starting to wear off after so much use.
I wish I had splurged and got more of the Medela bottles. The sealed wonderfully, the labels never came off and then all my storage bottles would match.
How many storage bottles will you need? The Medela PIS comes with 4 storage bottles. But most people need more. For my routine, I needed at least 8 storage bottles to make it through the day. And I liked to have enough bottles for two days so I could pack my bag for the next day without having to wait on the dishwasher to finish running. That's just me. Depending on how much milk you get, you could pour all your milk per session into one bottle and wash the other for your next session but that wasn't a good option for me. To start with, I would just get one extra set and see what works for you while you are still at home. Then before you return to work, stock up on what you need.
3) Membranes - These little puppies are what give your pump suction. Without them, you will not get any milk out. So, these are pretty important in my book. I always made sure to have a back up set in my bag. They are small and thin, so they can easily be lost or torn. Also, they will wear out eventually. When I noticed that I wasn't getting as much milk as I usually do, I would replace the set and I could definitely see improvement.
4) Freezer Bags - Milk only stays good in the fridge from 5-7 days, so if you plan to freeze your milk for long-term storage you will need to figure out how to store it in the freezer. I found the Lasinoh storage bags worked best for me. It seems like every sample pack I got while pregnant came with a few of these in it, so you can always try them out before you buy a whole box.
5) Breastshields - The PIS comes with 24mm breastshields. These may or may not work for you. I ended up needing a few different sizes throughout my nursing career. There isn't any easy way to figure out what will work best for you. It's kind of a trial and error thing. I would just use the ones that come with your first set and see if those work first.
6) Steam Bags - These came in handy when I needed to sanitize my pump parts and didn't have time to wait for the dishwasher to run. You just add some water, your parts, zip it up and throw it in the microwave for a few minutes. I didn't use these after each session (some people do). I usually only sanitized once a day, after my last pump session. These are also great because you can use one bag up to 20 times.
7) Hands Free Bra - This is a great invention. You can just strap this on and hook up the pump and your hands are free to do whatever you need. I did not have one of these but I want to invest for the next baby. I think it would have come in very handy so I could continue to do my computer work while pumping at my desk.
8) Car Adapter - This nifty contraption will allow you to plug your pump into your car and pump there. I usually had to turn my pump us higher than usual in the car but it worked just as great. This was perfect if I needed to pump on the go. (This isn't necessary if you have a Medela Freestyle). This isn't a total necessity, it would just depend on your schedule and if you will need it. I have heard of people with hands free bras that pump on the commute to or from work.
9) Bottles for baby - This is different for every baby. We introduced the bottle at three weeks. I started with the Medela bottles because it would have been great to only need one bottle but he didn't like those. Then I tried the cheapy Gerber bottles to no avail. So, I decided to try a wide nipple since that would be more like my breast. I tried the cheapest wide nipple first, NUK. They were a hit. We have stuck with those since then and they have done us great. There is no real way to know what bottle your baby will take. Again, it's a trial and error. I would start with whatever you want to try (only buying one or two), test it out and see. Just make sure to give your baby time to establish a good latch with breastfeeding but not too much time that they refuse to take anything but your breast.