Some basic computer audio questions

I'm running a Mid-2015 5K iMac with file player software. I'd like to hook up the computer to my full-sized stereo. The only direct out I know of is the headphone jack, which would mean a 3.5mm minijack to male RCA cable. 

Questions:

1. When people were using Mac Minis, is this how they connected to their audio equipment, or was it USB out to a d/a converter?

2. With the 3.5mm to RCA cable: are there any truly high quality examples of these being made? Most are included accessories or $2 cables. Even a few at $50+ look like repackaged cheap stuff. What's the maximum length a cable like this can go to?

3. I might move files to a mid-2012 Mac Pro tower. Is there any difference in sound between a SATA SSD and  blade PCIE SSD?

Thanks in advance!