IMO, none of the bikes mentioned. New, anyway. I see quite a bit of FG on craigslist. Some decent ones going for $150-$200. I'd try that out for a few months. It'll give you an idea of what you'll want on a proper bike or if this is even for you. I know plenty who tried it out, but have since sold their bikes for a geared one.

Also, buying used means you won't lose as much when you go to resell it.