My TV can decode Dolby and DTS. So I shouldn't have any problem in that. All I wanted to understand was that if big size 4K plays without any buffering and I guess it does play well. Have to spend some (& budget) sometime during the holidays for this project.
Yes, you can totally rely on this solution. Pi4 has USB 3.0 ports, so read speed is not a bottleneck. When I had this setup, my router only did 100 mbps, even then I didn't face any issue. But I'd definitely recommend a Gigabit router in case you're getting a new one. They aren't all that expensive, are available under 2k. But if you already have a router, even 100 mbps should be ok. Make sure you connect the Pi using a cable and not over Wi-fi. When I had it connected with Wi-fi, the streaming was not at all stable.
You can check Robu.in for Raspberry Pi. When I bought, some components were cheaper on Robu as compared to Thingbits. I'll also recommend getting the official power supply. Other than that, you'll need a memory card, heat sink, and a case with fan. The Pi4 easily goes up to 70 degrees in Indian heat, so it's not good for its long term health to run without fan.
For hard disk, I'd recommend getting something externally powered. Though the Pi can power a 2.5" hard drive, it just adds to the load on its circuitry. So better to avoid that.