Hi John. I am in no way a computer expert, but from my experience:

(1) Buy an external hard drive. They are not that expensive, and you can easily copy all your pictures/documents/videos/music to it. If you use "Outlook" or some other mail service, copy all your "Contact" information to it. Google the info on how to do the contact information. It is not that difficult. I would do this immediately. One day that computer will stop working.

Assuming your computer totally dies, having saved this info should not make it too painful to install on a new computer

(2) I would not suggest that you buy an estate computer and try to harvest the hard drive. First, it has to be an almost exact match to fit into your computer (both in size and operating system) and upon install, you may find you inherited some old problems.

(3) As to the question of if you can copy all your old information to your new external hard drive (including the operating system), I don't know. I doubt it. If people could copy operating systems to external hard drives, Microsoft would be out of business.