Though if drives are disappearing, that is first warnings that a drive is failing. To verify, use Linux by booting off either a DVD or USB flash drive to find out if the drives are actually disappearing. Another warning is a program not able to be installed. You could have a power supply problem and the only way to know is use another power supply. If you can't do Windows update, then download and manually install Windows update from Microsoft's web site.
Crystal Disk is not a diagnostic tool. It is a benchmark tool. Download the diagnostics utility from the storage manufacturer. In Linux, Disk Utility can run S.M.A.R.T. diagnostic tests like short and long. First run short and if it passes then run long. If it passes long, then run it again by starting with short and then long. I think running the tests at least three times to try to induce an issue. Seagate drives have a behavior of disappearing unpredictably when failure is soon.
Windows slowly degrades over time. A clean install is the only way to fix the degradation.
If you are going for a new computer, buy all new components and this includes the power supply.
The GeForce GTX 1050 TI is required because Netflix 4K requires the GeForce GTX 1050 to have at least 3 GB of video memory. This means you will need at least the 1050 TI. There are no benefits using M.2 for an HTPC. M.2 only works for storing virtual machines, 4K capture, video editing, complex graphics editing, and sending data over a 10 Gb network. Though how the M.2 is connected is different from the motherboard to motherboard, so the M.2 can take up an SATA port.