"There are a lot of raspberry pi displays around on amazon, but i guess not all of them will work with AIDA64 out of the Box?"
Yes, it works, power by usb 5v and "image data" by hdmi ... use as "second monitor" -> works fine and is compatible, use "normal" sensor panel and drag it to second screen. Many of them has a black case (and you can ever make a custom for that).
3,5pol, 5 pol, 7pol and bigger.
Just make sure they are powered by usb (mini or type c) and the video connection is either hdmi or mini hdmi. Another thing you can do is look for those displays that don't have a touch, they'll be a little cheaper - a little bit, but since the goal is not to use the touch, you can simplify it. All the cables you can hide into your case and optional, with the "frame", you can put it out of the case ando it looks well too.