Yeah, to use the built in QB inventory system you would want to record the initial purchase of the rental items as plain assets, then record the count as inventory items with zero price so it doesn't effect the financial aspects of the inventory system. To rent an item you would put the zero cost item on the invoice to reduce the count along with a separate line item for the rental fee. When the customer returns the rental items, issue a credit memo to up the inventory count. Of course this doesn't solve the scheduling problem. QB does have an extensive 3rd party ecosystem though, and if necessary an API.