Aren't you better off buying a disk shelf for your server rack? Those will hold 12 or 24 3.5in drives. You can often find them used for reasonably cheap prices, often pre-populated with old drives. This isn't bad, because even if the drives aren't useful to you, the sleds are. You can always on-sell old drives if you want. You can use SAS or SATA drives in a disk shelf, just don't mix them, stick to one type of drive per shelf.
For commercial use disk shelves always plug in to a controller unit that acts as the brains of the NAS, controlling the dozens of drives for the network. You don't need that tho. Home rack folks will often just use a PC with a HBA card with external ports (eg. SFF-8088 ports on a HBA card from LSI), and connect that to the disk shelves with SAS cables. Or you could buy the rackmount control unit used too. They aren't much, and if you want to upgrade from SAS (3Gbps) to SAS2 (6Gbps) or SAS3 (12Gbps) controllers, then they're modular upgrades.
The biggest drawback is probably noise from all of those drives working and fans to keep the HDDs and PSUs cool. Seeing as you're already going with rackmount tho, I guess that noise isn't a concern in your setup.
caveat: I haven't done this myself. I researched it and ended up passing because I can't manage the noise in my living situation. Otherwise, it's easily the cheapest and cleanest way to 100TB+ storage levels so long as you buy used gear. Need more storage later? Add another disk shelf to your rack.