Ok - that being said, I think you would still have the same tables. I laid out above. However, once an equipment has a child 'line item' record, it would not show up in the list for the next time you add a line item to an order. You still need a place to store these unique equipment assets. And, in order to link many of these unique equipment assets to one order, you need a join table.
So to implement this, create a summary field on the equipment table that counts the number of line items related. Then, create a formula field (text or checkbox) that has a formula something like: if([# of Line Items]=0,"Available","Issued") where if there line items is equal to 0, it is available, if it is not equal to 0, it has been issued.
Then create a report labeled 'Available Equipment' I would recommend only putting 1 to 3 fields on that report. Add a filter to that report: where [# of Line Items]=0. This report will be your active equipment list for use on orders.
Now go to your line item table form, and on the form element for the equipment reference field, have it display choices from your 'Available Equipment' report instead of the standard record picker fields.
That should do it.