Hardrocks are great for general commuting duty and esp as winter commuters. Fit some narrow slicks, a rear rack and fenders and you are good to go.
Its always good to have an additional bike but since you already have a good bike, you can take your time. If you wanted something faster and sportier but still practical enough for commuting, then a road/sport bike will do the job. Look for sufficient clearance for the tyres you want and possibly for fenders. You probably want some threaded eyelets for rack and fenders. Within your $500 budget you are probably better off getting something in the used market, either fairly new and little used or something old but very good.
If you want to do some loaded touring you will need something heavier duty such as a classic touring bike. Its always a debate how heavy you need to go.
Also budget for luggage, lighting and any upgrades you need.