Bear in mind that when configured as a surround speaker the Eras will be playing from behind you, rather than as a conventional stereo system would have the speakers in front of you (though you can adjust the balance of sound from front and rear to create a more neutral environment).
 

I would start with the Arc Ultra, but no surrounds. Try that for a while, for music as well as tv. It is reported to have better stereo separation than the Arc. Decide whether it’s good enough for you operating standalone.
 

Then, for music, consider the Eras in a conventional position, if you have space. The Sonos app will let you install them as surrounds, so you can try them in that configuration, or as a stereo pair. 
 

I would definitely buy an Arc over an Arc Ultra.  The Arc is 4 years old at this point, so in my opinion, it’s worth the extra $100 for that reason alone, in addition to reported improvements in audio quality. You may also find the improved dialog enhancement features of benefit to you.

The Era 300s are actually primarily intended for surround sound. They have both side firing and upfiring speakers.  They can be used as music only speakers (this is how I use them), but I would not say they are wasted as surround speakers at all.  The only reason I don’t use them as surrounds is because I really wanted dolby atmos music in a different room, via the Era 300s, and did not want to pay for a 2nd set of Era 300s...at this time.  I’m using Sonos Ones for rears currently, and they are good.  Era 100s should be slightly better, and Era 300s a noticeable improvement.  It won’t matter though if you are not watching dolby atmos content.

Sonos speakers don’t come with remotes.  For the home theatre setup, you can use your TV remote for volume, or the Sonos app on your phone.  All the speakers require a power cable, and the Arc/Arc Ultra will have an HDMI cable to your TV.  Nothing more.

Arc Ultra, no question there.

I’d really lean 300s too, much improved TV sound and more of it for a large room.

You didn’t mention a Sub but in a big room one or two can be very impressive. Gen 4 without a doubt, the older ones may prove limiting in the future.

Set everything up to use your home WiFi and you will only have power cords and the HDMI cable to the TV from the Ultra.

The Apple/Android controllers will likely run on anything (fairly new) that you have and the Web controller is getting pretty good too.

If you have a CD collection you like consider ripping it to lossless FLAC files (pay a grandkid to do it) and put them on an inexpensive NAS (Network Attached Drive) and have them instantly available with no CD flipping needed.

thanks for the reply.   definitively going with a subwoofer.   and like the idea of ripping my cds  but realistically that's in the same category as scanning old photos LOL!  appreciate the feedback!

Costco scanned our photos, they too live on our NAS.

We are old, so out-source the boring stuff and save your time for naps and Sonos listening.

Not mutually exclusive!

I’ve worked on ripping projects where I used several computers in a small area. My time was spent feeding disc trays and fixing Metadata issues. I could work through about 30 discs per hour. Now that the bulk of my library is ripped, I can add a few discs by stacking them beside my main computer and feeding the tray as rips complete. This hardly impacts my normal work flow. If work or family interrupts the ripping process, it’s no big deal. Eventually I’ll work through the stack.

I believe @melvimbe meant...”I would definitely buy an Arc Ultra over and Arc”.  

Yea figured it was a typo.   Arc Ultra seems like its probably worth the extra $100 just for the fours years of tech passage of time since the arc was introduced.