Riding miles with a heavy backpack is not recommended.
Get a bike with threaded eyelets for a rear luggage rack. Pick a suitable rack and fit some modern panniers with quick-release locking hooks. Make sure the rack is compatible with the hooks. A lot of rack makers do funny things, you need simple round section lengths for the hooks.
Avoid suspension.
Fit bolt-on fenders
Upgrade the tyres to a reliable commuter style with good protection.
Those drop bar, disk brake cyclo-cross/touring bikes are neat but at your price bracket, a simple hybrid style is better value.
Giant Escape City is fully equipped and a good price.