Whether offering $200 is out of the question depends on whether you really want it - there's always the danger of irritating the seller, who will then refuse to bargain with you at all on price. Personally, I think $300 to $350 would be a good price, and I wouldn't feel bad about paying the guy's $375 asking price, but of course that's me. It sounds like you're not sure about the C'dale, so maybe trying to get a better price on it is worthwhile.

If I were in your shoes, I'd definitely want to test ride the Dew before you commit one way or the other on the Cannondale. I'm only slightly taller than you, but the one I test rode last month felt cramped, particularly in the top tube length. Again, that's me and not you, but be sure you know how well you like the alternatives before deciding about the Cannondale.

As for pulling a kid trailer, commuting, and street riding, I think the Dew has really good specs, but personally I'd look at the disc brake versions because of the extra weight of the trailer. I was really interested in the Dr. Dew until I took a test ride, and found that it didn't fit the way I wanted. I know a lot of people really like their Dews, though, so I'll chalk it up to in idiosyncrasy of my body proportions and my expectations of what my riding position should be.