My PC doesn't have USB-C, so I'm looking for a PCIe card to connect and charge an iPad Pro so I can use it as a second display.
Is this possible, or is it only possible for a data connection via USB-C?
You can just use WiFi if you’re using for instance Air Display. I think Sidecar is out since that requires MacOS. If you just want to connect you don’t really need USB-C, you can use USB-A to USB-C cables, for best results look for one with USB 3.1 Gen1 or Gen2 for faster data transfer speeds.
Depends on the computer, but it can charge over that kind of cable, but it is slower compared to the included cable and charger (18 watts is what the included cable and charger can do, the max it will accept over a type-A source is 12 watts).