Tri-bars can be added to most any bike, my advice is to buy something off craigslist that is in decent shape or use bikesdirect.com; I know price is an issue for you, but believe me....if you spend a little more you'll be able to grow into the bike and then in two seasons not be thinking about buying another one....if you don't need the bike right away, SAVE for it....make it an early Christmas present this fall....but do not settle! You'll appreciate spending a little extra on a bike with at least shimano 105 on it like this:

http://www.bikesdirect.com/products/windsor/fens_xi.htm

I have this exact same bike that I keep at my in-laws in Arizona so I have something to ride when I visit....it's a little more money but a much better component set up than the dawes you referenced....if you're hung up on the $500-600 price point then consider something with Tiagra shifters incorporated in the brakes....while the Sora stuff pricepoint is good, shifting them sounds like a meat grinder and doesn't have the most fluid of motion.

We'd all rather see you get a bike you can appreciate with a couple extra bucks than one you regret later on....if you want more info / help let me know and we can PM, email, or even phone chat or text.