I went with a lower end Janome machine. Absolutely love it. I would be a "little" concerned about a used one only because if someone didn't use it nicely things can get bent. I haven't run into anything like that but I have heard stories.
With that said, as long as they can show you how to thread (the most important and usually what gives some people hissy-fits) and it works well, go for it.
I don't use mine often, but when I do it is worth EVERY cent as it has saved me a kajillion hours of time
Good luck!