As usual Mike is correct, bit to add to it:
SDK stands for Software Development Kit
It's purpose is to make it easy for developers to create, test and de-bug software to be used on the iPhone.
I was thinking about buying the original iPhone
I still have my original and I love, but here is the thing, Apple has just announced iPhone OS 4.0 and as with almost everything Apple does, it will be moving forward and as such leaving some devises behind. (Snow Leopard can only be installed on Intel Macs, as will all future Mac OSs) iPhone 4.o will official drop support for the original iPhone. Now it will still work, but it ill no longer be updated or have features added to it, and very likely it will soon have no software that can run on it.
Get a iPhone 3G at the least, but as there are heavy indications that an new version of the iPhone is coming out, you may be able to get a 3GS for relatively cheap soon.