How the computer is configured whether you buy off-the-shelf or build your own is important. I built my own home computer and I think it can do just about everything you described except I’m a 2-channel guy.
I do listen to music stored on my computer’s internal HDD.
I have a pair of Audioengine HD-6 speakers I picked up used hooked up to my Windows PC running Windows 10. I don’t push the music out to my 2-channel audio system or my TV.
In terms of ripping and storing, as long as you have a decent optical drive and sufficient disk space - no problemo. I would suggest an external drive for storing your music files but that’s a matter of personal preference.
Music files depending on format, (MP3, FLAC, AAC, etc), can take up a lot of disk space, so you’ll need to take that into consideration when looking at Hard Disk drive capacity. SSD’s are faster but are more expensive.
What are you comfortable with: Windows or Mac? Decide on the operating system. Decide on the computer’s form factor: e.g. Desktop tower, mid-tower, low-profile, all-in-one, etc.
Everything you are attempting to do is doable, configured properly you can even use an iPad to access streaming services and play the music through your home audio system.
You mentioned an HDMI cable running from your stereo to the TV. Do you have cable or satellite? Do you also have a HDMI cable running from the TV to a cable or satellite box?
Check the related topic threads here on the Gon or go here:
https://www.computeraudiophile.com
Not sure where you live but I’ve seen a McIntosh setup at the "House of Music" in San Francisco http://www.houseofmusicsf.com/ that does everything you’ve described. Since you own the 2125, maybe give them a call or shoot them an email. They’re a McIntosh dealer and should be able to answer a lot of your questions. They also carry NAD.
If I can answer any more questions or point you in the right direction, let me know.
Good luck!