Thunderbolt 2 can support a single 4K monitor and they still make TB2 external storage devices. Apple makes a bidirectional adapter that will allow some 3&4 devices to work with a 2 port but every device is different and most will not be officially supported. Link to adapter is https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter

I would not add a TB2 card to an older machine though. Just find an adapter that works with the fastest port on that machine. There are plenty of devices that work well with usb3.