I think it's preferred if you use a USB sound card with "line in" instead of "mic in" levels. That way you don't need to attenuate the higher level coming from the rig.
Yes, regular 1/8" stereo cables especially if you are using "line in". If you only have "mic in" you may need an attenuator.
I might recommend an Intel NUC if your software needs a little more compute power than the Raspberry Pi can provide.
Almost all small form factor (Raspberry Pi, NUC, Laptops) solutions are a PITA because they do not natively have a line in jack for audio.
Mini-ITX stuff will usually have a real line in jack but may be a little bigger than you'd like.