As you have found out the range in quotes for a service vary tremendously. There is Subaru's maintenance that is a minimum, there is Subaru's severe maintenance schedule and then there is preventative maintenance that may be suggested to make your Subaru last. Maintenance all depends on your driving conditions, vehicle and your how long you expect to keep your car.
Make sure when you are comparing quotes you are comparing apples with apples. One quote may be cheaper but leave out replacing the fuel filter, another may be cheaper but leave out checking valve clearances. At the end of the day you hope you are getting quality workmanship and getting what you are paying for. Put together any service records you have for your car.
All the maintenance does not need to be done at one time. Perhaps if you don't know much about the previous service history of your car, go to the repair shop you felt most comfortable with or had personal referrals from friends and workmates. Bring your service records to them ask for an inspection, oil and filter change and ask them to prioritize what repairs and maintenance you should do.