Yes, a Port would be appropriate. The Amp would be only if you wanted to power some non-Sonos, non-powered (old style) speakers.
To your other question, yes to the first part, no (well, not easily) to the second. You can definitely set up the Era 300s for music only use, with no issue, but the Sub would need to be ‘moved’ in the software from the Arc’s ‘room’ to the Era 300’s ‘room’, something that is possible, but gets onerous quickly. And if you use TruePlay, it would be wiped out each time you moved the Sub from ‘room’ to ‘room’.
Yes, a Port would be appropriate. The Amp would be only if you wanted to power some non-Sonos, non-powered (old style) speakers.
Great Response!! However, I take issue with your use of referring to non-power speakers as “Old Style”. I may be old (being a bio-logical product of the 1950’s) but my $600 pair of Definitive Technology D7 Book Shelf Speakers (purchased in 2021) I use with a Sonos Amp w/turntable and sub; are far from “Old Style”. 
Happy Holidays to You!
Not much older sir, but I am a child of the 60s…but only by a week from the 50s…;)
Happy Holidays to you and yours!
If you get one (or more) 300s you can also get (just one) line-in connector and skip the Port purchase.
300 and adapter seems a good deal versus the Port’s cost.
