My recommendation is to partner up with a friend/acquaintance you trust who is an avid cyclist and ask her/him to peruse Craigslist and FB sales groups for/with you. And, if possible, ask that person to tag along when you go look at a bike. I'm a pastor and have done this with 10-12 persons/families in my congregation (one family has picked up five bikes with my help in the past few years). You can describe what you're looking for and you friend can look for things that match. You would be amazed what you can pick up for $600 working with someone who knows bikes.