What type of bike is appropiate for my situation?
If it were me I would buy a comfortable steel road bike with fenders and racks. This will give you a bike you can comfortable go distances on as well as practicality for the commute. You aren't racing so you don't really need an extremely expensive, high end road bike and a steel frame will be comfortable and durable assuming you store it properly. When you feel the need to romp around you can take your MTB out. If the road bikes are out of your price range then consider a similar setup with a single speed/fixed gear. Fixed gear bikes will make you pedal smoother but a disciplined rider of any bike can get the same benefits using cadence drills without the added danger of getting bucked by pedals moving faster than you can.