You can use any win7 disk, as long as it's the same version( Starter, Home, Pro, Ultimate). If SP1 is not included, the laptop will download it form MS with all the other security updates. Important: make sure you have all the drivers for the laptop on hand, before you start.
2. It should, its most likely an OEM key which will work again given its being installed to the same hardware.
3. Means you will have to update it when its installed, but thats not an issue.
4. You dont, any tech store I would think.