Not really, no. Frame rate at 4K resolution will not be good, you can forget about transcoding. 1080@60 already brings it to the limit, so even if you clock it high, the RPi 3 can't deliver a usable framerate at 4K.
I see no reason why you should necessarily need this microSD. The only important thing is that they are sufficiently fast. Das sollte auf alle unter https://www.galaxus.ch/de/s1/producttype/speicherkarte-61?opt=v16-460:32000000000&rng=16-443:800000:|16-443:400000:800000&pdo=16-697:3460&tagIds=535 zutreffen.
You are surely aware that Windows IoT is not just a Windows 10 for the Raspi?
A Raspberry Pi is not suitable for NAT. Unfortunately, we cannot tell you what you have to pay attention to on the PC without more detailed information.
This means that it is not a standard mainboard (form factor), such as for PCs, or a socket for the CPU. The latter is permanently installed or soldered and cannot be replaced (onboard). The product is indicated in this way because it is listed in the same category as all the other PC mainboards, for example in ATX format. You can basically ignore the specification for the Rasperry Pi, this is more for the search, in case someone uses the filter in a product search.
See specifications:
Mainboard Features
Form Factor Special
Processor socket Onboard