I hate to say it, but from your posts it seems like you don't know enough about bikes to make an informed decision on an old used bicycle. Don't worry, we were all there once. Buying a new bike is a good idea, but if you're poor/cheap like me you don't wanna drop $200 on a beater to ride to school.
What I did back when I didn't know anything was buy the cheapest useable bike I could find. I paid $10 for an old Firenze mountain bike. I learned how to do minor repairs, and then slowly worked up to more major repairs. It's an inexpensive way to learn. I only stuck about another $10 into the bike for cables and other misc stuff. Just make sure you buy a bike that is complete and rideable. The nice thing about buying a really cheap bike is that if it's complete garbage you're only out what you paid for it.
Then once you know more about frame material, proper fit, etc you can start looking for a nice old road bike to convert. Honestly, if you can't do basic maintenance I wouldn't suggest you jump into a FG/SS conversion. You'll be in over your head and probably get frustrated and give up.