If you aren't going to spend a boatload, then wirecutter.com is a great resource got good modest prices stuff.
If you have an older system, then you only need upgrade your AVR really for the video signaling (that is support for UHD signaling) and to handle digital audio via HDMI. Net, net, replace your AVR if you do not HDMI 2.0 capable receiver.
- Yamaha RX-V381. This is the lower end version with 5.1 and seems to be more of an Amazon favorite at $280 and is 5.1 as well without upscaling (which you do not need typically as the scaler in the TV is usually pretty good).
- Denon AVR-S510BT is a basic 5.1 channel system that cost $200. It does not internally upscale. This should be a problem since the television does this as well.
If you want a bigger 7.2 system, then you need to buck up to a bigger system:
- Denon AVR-S720W is a 7.1 channel system. It is $480 only. It has HDMI 2.0 for full UHD. However note that it has a reliability problem with the headphone jack incorrectly detecting a headphone and shutting off the main phone.
- Yamaha RX-V679. This is the Amazon best seller at $380. Appears to have roughly the same feature set but be reliable. Upscaling is the main feature
- Yamaha RX-V681 which is $600 and has Atmos and DTS-X. The main complexity with using Atmos is that you need down firing speakers or something fancy to get vertical sound.
The specific cables you need you can get the basic one from Amazon or Monoprice and they don't fall for paying a zillion dollars for it.
- Monoprice Ultra Active HDMI with 18Gbps for $11 (order more given the shipping charges) or $12 from Amazon.
- Amazon Basics for $6 gives you a functional Toslink optical out cable
- Speaker wire. Do not be deceived, you can a decent set for $15 from Monoprice.
Then you need speakers sets, these two have gotten much cheaper. Personally, I think sound really makes:
- Pioneer SP-PK22BS. These are a small bookshelf speaker set for $450
But if you want really high quality then it is better to put together a system, so wirecutter.com recommends the ELAC series. Note that if you have an existing system, then you want a modest update, just make sure that you have a good center speaker, that's the key dialogue speaker. You want something that is going to sound good, so here is a good one:
- ELAC C5 Debut ($180 at Amazon). This is a super reasonably priced center if you have all the other parts, this is the most important upgrade.
- ELAC B6 Debut ($280 each speaker at Amazon). These are tower speakers for the left and right. You need two
- ELAC S10 Debut ($200). This is a 10 inch subwoofer
- ELAC B5 Debut ($200 for the pair). These are bookshelf rear speakers.
For really good sound, they recommend:
- KEF Q-100. These are bookshelf left and right speakers for $300 a pair. These are right now at a huge discount from their normal $800 price.
- KEF Q-200C. For $500 a center speaker. The center is generally the most important given that this is where the dialog is.
- KEF Q-400. This is a great subwoofer for $400. These are at a big discount as well.