Look at the "Advice for New Commuters" sticky at the top of this section. Unless you plan on also riding on rocks and mud, a bike with thinner tires (32-35 mm) and no suspension will give easier pedalling. 8 miles each way will be nice once you are in shape, but it may be a good plan to only do it 2 or 3 days a week when you start. Budget for spending money on spare tubes, basic tools, water bottle and lights. Good luck!