I would really recommend you buy one. I exclusively breastfeed (no formula) and have always done, but for sanity, I do need to leave DD sometimes and therefore she has to have milk so I need to express to keep milk in the fridge or freezer.
If you don't have a pump and have no way to get milk out and don't use formula, then you'll have to think about the times when you need to leave LO for a job interview, the dentist - things you really can't take LO to. If you won't have any of these, then you also need to think that sometime (I promise this will happen!) you'll want to leave LO for some 'you time', even if its only a couple of hours a month to go to the hairdresser or to the shops.
I got my Tommee Tippee Freedom breast pump (manual) from T K Maxx for £12.99 and they are also often on offer in Tesco too (I think its their baby fair this or next week too so you could probably look for a bargain).
However, I do know a breastfeeding advisor who only ever expressed by hand over a sterilised bowl, so it is possible, if you'd prefer this option.
That's a bit of a guddly answer, but I'd recommend buying a cheap one (but not to express for at least the first 3 weeks to let your supply settle down) and seeing how you get on with it - then you can always invest in an electric or more fancy manual one when you are established with feeding and know what you want.
Valentine Xxx