Many new sewing machine mechanics ask "What parts should I keep in inventory?"
The answer is as little as possible to keep overhead down, but keep those parts most commonly needed on hand to speed up servicing.
Many people think of gears, levers, springs, shafts, or computer circuit boards when considering parts, but these are seldom required. One of every 100 machine will require a major repair. Ten out of a hundred will need more common parts. The rest just need good thorough cleaning, adjusting, and servicing.
In the picture above, you see needles, spool pins, spool caps, feed dogs, tension discs and associated tension parts, bobbin tires and bobbin winder parts, light bulb, bobbin case or carrier, presser feet, hook, and hook bracket.
When you begin to establish your inventory, these are what I recommend. As you begin working with machines commonly in use in your area, you can add more of the parts you come to expect needing.