Your questions show that you are on the right track. Take a look at the Invoices starter solution that comes with your copy of FileMaker. (If you are trying to decide whether or not to buy FileMaker, download the free 30 day trial copy.)
Each Invoice in that example could be a work order. And the portal to a related table could serve to list all the parts on your work order. The basic relationships (Though the table names are different in the starter solution) look like this:
WorkOrders::__pkWorkOrderID = Parts::_fkWorkOrderID
Inventory::__pkInventoryID = Parts::_fkInventoryID
You can place a portal to Parts on the WorkOrders layout to list and select a Inventory record for each given WorkOrders record. Fields from Inventory can be included in the Portal to show additional info about each selected part and the _fkInventoryID field can be set up with a value list for selecting parts from inventory by their ID field.
And using these same tables/relationships there are also other ways to list all the parts on a work order (I did see that "up to 400" figure), that do not require a portal to do so, this is just the "get started" example