As johnny has said, buying a guitar can be a very personal thing, an SG might look great but sometimes you don't get the 'feel' of a particular guitar, no matter how good the name or how expensive. Is it a surprise? If not, why not visist a few guitar shops and try some out, you might even get a good deal. If you are near enough try Denmark Street in London for a wide selection, then check out your local stores, ebay, Loot, local papers etc and see what sort of price you can expect to payin comparison.
Ebay can be ok, but you don't really know what you are getting and whether the guitar is a genuine Gibson, lots of cheap copies out there with fake decals.
The pickup is the device under the strings that transfers the string vibrations to electronic sound through an amplifier (is this part of your buying idea as well?) Some are single coil pickups (SG has 3) and where you have two together, they are usually called humbuckers.
You might want to look at some of the info here:
http://www.everythingsg.com/