I moved to a couch home theatre PC setup years ago and it's great. I'm sure you'll enjoy it!

For a keyboard, I recommend the Logitech G613. Mechanical keyboard, runs on AA's, gaming quality (don't need to worry about n-key rollover). No RGB lighting, so better battery life as well.

For a mouse, I like the Logitech G603. Gaming quality, uses 2 AA's for great battery life and a nice feel.

Don't go too small with the TV! You'll likely need to either lower the resolution or up the dpi scaling to read text properly when you're at a comfortable couch distance back. I use a 65" high end Samsung LCD myself. The 120 Hz refresh rate and VRR work very well with my GPU. Though I occasionally have to manually set older games to a 60 Hz refresh rate to avoid breaking them. Side note: Samsung TV's don't support Dolby Vision, as they use their in-house HDR 10+.

You can make a lapboard from 2" thick styrofoam boards wrapped in keepfreshphane or whatever it is called locally (that transparent plastic wrap to keep food fresh).

A wireless keyboard and mouse with descent wireless range.

A wireless gamepad with good range (get a Logitech Rumblepad 2 if you can, they're the best otherwise the Xbox 360 gamepad, I'm a bit on the fence of the Xbone controller and the F710 is rubbish).

A kingsize tv (I use 55") bigger generally is better.

A surround speaker setup, maybe with a good surround reciever but be careful it supports at least 4k or 8k if you want use HDMI to be futureproof.

As for the pc itself, anything you want to spend on it and how much performance you want out of it.
Personally I have a very modest pc, cpu and mobo is 10 years old, gpu and ssd 5 years.