At that capacity, I’ll cast another vote for SSD if at all possible, but you can certainly get small HDDs pretty cheap now.
If you want the easiest and cheapest way to add more drives, do a search for “sata port multiplier”. These cards go for around $25US on Amazon or Ebay. They are NOT fast! It uses a single SATA port to run up to five drives, so all the drives split the bandwidth, but long ago I ran some of them for a few years without any problems. You simply run a sata and power cable from your motherboard to the card, then plug in your drives, it doesn’t even require a slot on your motherboard.