Apple developer account and upgrading the OS
Q1: for iOS development that is the place to start.

Q2: No, you will be able to download the latest versions of iOS, including the current beta versions of it. For Mac OSX beta versions you need to register as a Mac Developer at http://developer.apple.com/programs/mac/. With this registration you'll not be able to submit iOS applications though.

Q3: Yes, you can register an account even from a Windows machine.

Q4: See Q2. I don't know if you'll be able to download non-betas with a Mac Developer Program Account. Apple's page only talks about Pre-release Software, not existing software (besides Xcode).

Lion at the moment is only available through the app store which is only available when you have the Snow Leopard installed. I'd suggest picking up a DVD of Snow Leopard and then update normally to Lion through the App Store.