Since you are still shopping, I will give a little recommendation of products that worked good for me so far.
I recently added an external drive to my pi 4 for a place to put my 24/7 recordings. It was tricky to find a good external enclosure that had its own separate power supply and could also be recognized after a power loss or reboot. After much tinkering and a firmware update, I was able to make this Fideco and 6TB WD Purple work for me. The original problem it had was that the stock firmware was so old that if I rebooted the pi while leaving the enclosure powered on, the usb communications would fail.
As for cameras, I have been playing with Amcrest lately. I have moved on from using the cheapest chinese no-brand cams to something that costs a little bit more, around $60 USD. I was pleasantly surprised at the quality and performance. They actually have a web interface that does not require internet explorer or flash player for viewing. They can also handle multiple connections. The cams with audio are already encoded with aac, which makes stream copying them much easier and use less cpu load in ffmpeg. cams tested: 5mp poe no audio, 5mp poe with audio
