Hi knivesforhire!  As far as I know, they are generally considered to be good typers. I have one that is a 6 cpi, so I don't use it much for writing things.  They are plentiful and easy to find on ebay and at antique malls/flea markets (not rare at all, is what I'm saying).

What is your intended purpose for this machine? Do you want to fix it up to use, or are you wanting to fix it up to sell? 

For just learning purposes to fix up and use, this would be a great first choice, since you don't have to worry about messing up. You can dig into it and learn all about how they work and how to clean and do minor repairs.  Lots of people have them, so advice will probably be easy to find.

The Classic 12 is one of Smith-Corona's better manual portable typewriters.  I own several, including a very large-print teacher's edition made in 1967.  Reading around in the posts has it that you have an escapement problem in the one you have.  You're in luck, because you can still find lots of Smith-Corona portables lying around here and there, and you could pick up one for a few bucks.  The escapement mechanism has been fairly standard throughout a few decades and is used in both the manual and the electric portables, so you should be able to find something as a "known good set."  Then, you can decide if you want to repair the one you have using a second one to look at how it works, or if you want to fix the second one and use your first one as parts.