We have started buying these at work now:
https://hengedocks.com/products/tethered-dock
Seems a lot more stable than the Belkin docks that Apple sells.
I'm not sure about buying a monitor off Kijiji. You will want something that has HDMI in, and there may not be a great used market with HDMI. If you look you can find 24" monitors as low as $150.
https://www.memoryexpress.com/Products/MX62119