I would suggest having the possibility to add multiple parts for a single step in a procedure in order to have an estimation more accurate (or actually useful, because we can't even use the feature as it's limiting to single part) per step.
I would definitely explore the use of Bill of Material (BOM) that could be useful for specific to pmp or specific to pms (because sometimes the procedure is the same, but based on the equipment, as example, filter sizes can differ).