For home lab/hosting you can get really good performance out of used/refurbished mini pcs, look for something like hp mini 400 g4. They can be found for around $100 to $150 on ebay. They generally fit a 2.5 inch ssd drive as well as 1 or sometimes two m.2 ssd drives. I find that intel nucs are pretty expensive compared to the specs, like you’re paying a premium for the brand and form factor since it’s a bit of a niche market. These mini pc’s are more business office oriented and are way more accessible at a lower price.
I realize you’re asking about storage, you could go the route of turning that old server into just a dedicated nas and use other hardware for your services/compute.