Ideally, if using an external keyboard and mouse, you would choose Bluetooth (wireless) devices - as these don't require a wired connection. That said, your iPad does support USB keyboard and [basic] two/three button + scroll-wheel Mouse devices.
Your iPad9 has a single Lightning port. If you are using an Apple Lightning to Digital AV (HDMI) Adapter to connect your monitor to you iPad, you'll not a have an available USB Port for simultaneous connection of wired USB peripherals - and for this reason Bluetooth devices add flexibility.
iPad models that feature a USB-C port instead of Lightning are more flexible as USB-C provides opportunity to simultaneously connect multiple USB devices via a hub - and/or other interfaces such as an HDMI or DisplayPort monitor.
Simultaneous connection of HDMI plus two or more USB devices via your iPad's Lightning port will be challenging - as you will need to identify a third-party Lightning Adapter with appropriate ports. Economically, other than considering an alternative model iPad that features a USB-C port, you find you have more options by considering a compatible Bluetooth Keyboard and Mouse for your application.