@SergSlim Welcome to the Forum. I agree with building piece by piece but make sure you have a focus on what the ultimate system goal is going to be to ensure the AVR (Receiver) will meet those needs.
If you want to compare Receivers try this:
Denon 2800 vs 970 or just for grins I did
Yamaha RX-V4A vs Denon 2800 small differences in power output are no significant enough to call one unit better than another. I'd go with
@Eppie suggestion on the Denon 3800 if I found features it provides are what you are looking for now, and possibly down the road. (I'm big on purpose buying.)
I also think Eppie's suggestion on the Polks is right on target at this price point. Check out this video from Erin's Audio corner.
However, given what's available to you, and my experience with Klipsch Subs (I have 4 older Klipsch Subs all around the house as I have 3 Home Theater systems - and Yes, I know I have a problem) is they never die. If they did, I'd step up to a RSL 12 Speedwoofer, or perhaps a SVS or HSU for better accuracy. (My Misses would probably have a heart attack if I started replacing them if they were still functional.)
Here's a bit of a story on my 5th Klipsch Sub. (Did I say I have a problem...?) After reading the
Klipsch R-121SW Audioholics Review by
@shadyJ I stumbled across one at a Best Buy Auction returns site, and got it so cheap I couldn't resist it, so I bought it. I set it up in my Man Cave /Junk room in the basement, and after letting Audyssey adjust it on my old Denon AVR-S900W Receiver, it's amazing. So while there are better Subs out there, if you find yourself up against it budget wise, this can be a decent solution as the Room, and it's position in it, has a tremendous impact on a Sub's performance.
I'm thinking that going from no system, to a full Atmos setup could be jarring => especially for your neighbors, wife, etc. Buying the Polks and a Denon AVR would be a great way to ease into it. Good Luck with your choices.