>>1102066>my case doesn't support multiple hard drivesThe case is not as much of a hard limit. As long as you have a free SATA port on your mainboard you can add another drive. You just can't screw it into the case but you could put it on the bottom or beside - even put it on an edge to save space as long as you don't accidentally bump it, maybe on a rubber mat to decouple vibrations.
>I ideally want at least 8TB of space for now, so what is my best and cheapest option with only storage in mind?That depends. Does all your storage have to be accessible at all times? Then a NAS or fileserver is not a bad idea for adding that much space.
If you don't need everything all the time you can buy an HDD docking station. It's a small box that sits on your desk that has 1 or 2 slots in the top where you can plug in bare-bones HDDs (picture just a random example). You connect it via USB3 or eSATA. It's a 1-time investment of 50 bucks that turns any future HDD you buy into an external drive like a USB thumb-drive. It scales very well and is future-proof since to expand you only need to buy a cheap bare-bones HDD. Then you just plug it in to access it. Added bonus is while in storage degradation is slower since the HDD just sits unplugged on a shelf instead of going through powercycles and spinning all the time. And you can continue to use your old HDDs as well.
I switched to a docking station about a decade ago and it's a very convenient solution to mass-storage. What I access regularly I keep on my internal 2TB disk. For backups I plug in an HDD, then I might want to watch a movie so I hotswap to the movie HDD, then later an HDD where I keep TV series, etc.
>SSD, but is that a smarter choiceNot for space since money is a concern. SSDs are still considerably more expensive per TB. But performance-wise putting your operating system and programs/games onto an SSD is by far the biggest boost you can probably get for your system. 200-500 GB should suffice for that job.