Keep in mind that Black Friday and Cyber Monday just happened.
It's worth learning from an organization that does the work of preservation of paper on a truly massive scale:
"The 4RU chassis can be fitted with 106 3.5-inch hard drives..."
106 x 29 = 3074 terabytes
2.5 petabytes = 2500 terabytes
3074 - 2500 = 574 terabytes
574 / 29 = 19.79 so about 19 or 20 drives out of the total 106 used for (parity? hot swap?).
I have a feeling that this might be slightly out of my budget.
Online, you aren't getting 6TB+ for under $100 without going used or very sketchy. The best I see at a reputable vendor like B&H Photo is a new 6TB external WD for a Black Friday sale price of $130. Just look at https://shucks.top/, what you want isn't happening.
Watch your local Costco, Best Buy, Walmart stores - you might get lucky if they put out a palette of externals for a special price.
You cannot connect a SAS HDD to a SATA port with any sort of cable. You need to install a SAS controller card into your computer, and consumer PCs and Macs don't normally have those.
The reason you see so many SAS refurbished drives for so cheap is that people like you mistakenly buy them and discover they can't make them work on their computers, so they return them or try to fob them off on eBay and the like.
50GB x 30 days = 1.5 terabytes per month x 12 = 18 terabytes per year x infinite number of users = updated terms of service that states "unlimited storage BUT NOT LIKE THAT YOU UNWASHED HORDES OF HOARDERS".
A quick Google search and I see some of the clickbait YouTube conspiracy videos you might be getting sucked into. Stop watching that brain-rotting nonsense, it's just somebody's meth-fueled fantasy.