Can I purchase Kit items?

I've set up a kit/package containing a number of items that are used to assemble a larger piece. I want to purchase a kit from a supplier but it isn't appearing as an item I can select on a Purchase Order.

Are kits purchasable? Should I be using Assembly or Item Group instead?

I'm struggling to think of how to solve the issue of, we buy a Control Unit for which the parts come from multiple suppliers. How can we manage the inventory surrounding that top level item and can we purchase the kit without the wires or with extra wires if we have leftovers to be used in our warehouse or a lack of?