Need help fast

I'm going to my audio place in the am to get a quote. Got a 2013 ecsb with factory Bose and nav. I want to keep my factory nav head unit and add alpine speakers to my doors, an alpine amp and one or two 10 inch subs. On the phone he said they would use a line out converter? Do these produce good sound? Will the bass go down the louder I turn it up like the Bose? And should I add a capacitor to save my alternator. Thanks