Is there a typo on the HDD prices? Looks super expensive. With that amount you could have 48TB of NVME drives (assuming you have a controller for that many sticks, but you probably could buy one with the leftover from the sticks)
In general you shouldn't spend more on newer/larger capacity HDDs unless you absolutely need that much storage per slot (that is, you are space constrained), as the prices invariably go down in less than an year. Smaller drives with 12~16TB are a much better TB/$. Check diskprices.com to see current trends.
If you care about speed, relying on HDDs for that is a bad proposition. You are better served by a SSD cache in front of the HDDs. In case of SSD failure, it's much cheaper/faster to replace that SSD than to rebuild your strained RAID.
As for CPU performance, pretty much any newer Intel QuickSync will handle your needs. If money isn't an issue, I'd rather go with Intel NUC 12th gen or newer. I have one and it handles anything I throw at it, including the most demanding 4k transcoding. The NUC and a 4-disk Synology uses less than 50w with the disks being used.
You can do both. Sell a license and sell cloud storage.
The price depends on the market. Some people won’t pay out of principle, and some might even think it’s too cheap if you provide enough value.
The only way to truly know is to actually build it and sell it.
Best of luck!