The manufacturer requires timing belt service at 105K miles as you know. The dealer is correct in that if the timing belt breaks the damage would be extensive and expensive. However, the service is not yet required as far as mileage is concerned. What you need to do, though, is check to see if there is a TIME limit on the service in the owner's manual - that is, whether Subaru says 105K miles or XXX months. If there is, and you've passed the time limit, then the timing belt service is required.
All parts required are available, including the coolant conditioner, from the dealer's parts department.