the Diamond Back could work fine but it probably has been assembled with little attention whereas a bike at a bike shop has. What you're saving in price now may come back to bite you in maintenance issues like a rear wheel that needed some tuning up or early replacement. Your understanding how to maintain the bike will matter more than the price you're getting.

So first step isn't getting the bike. First step is educating yourself about what is a good fit/posture on a bike, then a realistic assessment of the bikes intended use. New bikes in the low price range pretty much all have straight bars and will need another $30 or so in bar ends and tape. Most drop bar bikes are racing bikes and a smaller number cyclocross bikes, prices upwards of $1000, there are a few dedicated dedicated touring bikes but not many bike shops carry them. Lowest cost rack and panniers is about $75 with $150+average.

A bike that can take 32mm tires is a good basic touring size for carrying weight with larger tires appropriate for heavier riders and heavy loads. A light bike that rides nicely unloaded may not be that great for carrying a load so that's where you're going to rely on others opinions if you can't test ride a loaded bike.