Customer could see how many are available and reserve units

A customer will be renting recreational equipment (like bicycles etc). 

"If a customer wants to rent 2 units 12pm-4pm April 6, they could see how many are available and reserve them".

Is something possible within a plugin or something? 

You can achieve that using any of these widgets.

https://widgets.jotform.com/widget/quantity_gift_registry

https://widgets.jotform.com/widget/inventory

If you want to pass it to the payment tool, you first have to perform a calculation. After that, you can pass the calculation to the payment field to accept online payments.

If you need further assistance, let us know.