Tiagra is totally fine and the Trek 1.5 is a good bike as it is. Don't waste your money on 105 or Ultegra parts until you wear out the ones you have already: the differences in performance are minimal. If you've got money burning a hole in your pocket, buy a Garmin 500/510 with heart rate and cadence sensors and start logging your rides on Strava. Buy good lights and perhaps SKS fenders if you're going to ride in the dark and wet. Perhaps upgrade your tires, but even then wait until you wear out the ones you have. Better brake pads may improve stopping power, but by how much depends on how bad the ones you already have are. Buy a bike workstand and some tools and learn how to repair and maintain your bike. If you still have the bug after a year or two of riding your 1.5, buy a nice new bike and keep your Trek as your winter/beater bike. As for now, just get out and ride.