I have a ThinkCentre m90q with an i3. It’s a few years old. It’s a lot more powerful than a Pi. A Pi will not cut it.
You will preferably need something with modern hardware encoding. Support for h265 and AV1 is a requirement nowadays to play high quality sources and find anything for newer stuff. Moreso if you want to watch 4k content.
My grandma uses WhatsApp. The only reason I’ve ever used SMS is to receive a verification code