If you do find it at a used book store, there's two things to check.
First, although the errata is available, unless the difference between new and used is greater then the price of the PDF, I would not recommend getting it. The first printing (the color one from AEG) has badly layed out gear tables. So badly they're almost unusable. it has a good bit of errata, but that's avaialble from the Crafty Games site. Second printing (black and white from Crafty Games and Mongoose, look on the back for the logos) fixes that errata and the tables. It is from the in house printing era at Mongoose though, so the binding will fail sooner or later, probably right around page 16. The US printing is slighty smaller, slightly lighter, and has a much better textbook style binding. The main thing to check for this on is that the spine is slightly rounded and if you look closely at it 'edge' on, you should see signatures rather then a stack of pages glued in place.
As far as supppliments, I would recomend getting World on Fire at the least. It presents the setting from the CCG, but it also adds two base classes, some expert and master classes, fills out the melee and unarmed comabt feat chains, clarifies and expands the contacts rules. After that, it depends where your intrests go.