I am fortunate, I have 2 parts supply very close to my service area, otherwise I order online from Repair Clinic or Appliance Parts Pro. They supply parts at a technicians price if you ask for the discount. As far as what I carry in my vehicle, it comes to you as you you get experianced at the jobs you do. Switches, water valves, ignitors, fuses, T-stats, ice makers, etc.
There's an appliance blue book for under $200 for price quidelines. I deviate from it for the most part like 10% less cause of the economy I live in. It stil a good wage if you manage your business expenses accordingly.