My bulk storage box is a used Dell R730xd, and after going through the process I’ve got a short list of things I wish I’d known going in:

• Check iDRAC Enterprise licensing before buying, not after. A lot of used listings have it stripped or expired, and losing real remote management is a bigger downgrade than it sounds like on paper.
• Drive caddies are where sellers actually make their margin. Sometimes a handful of caddies costs more than the drives you’re putting in them. Either buy fully populated or budget for caddies as their own line item.
• Idle power draw and fan noise are datacenter numbers, not homelab numbers. This thing alone can pull more than my whole mini PC cluster combined, and it is loud. Figure out where it physically lives before you figure out what runs on it.
• Being out of warranty is a fine tradeoff for a storage/backup role (worst case is a resilver), a worse one for anything that actually needs uptime.

Worth it in the end, ZFS on this thing runs circles around anything in the mini-PC tier, just went in more clear-eyed the second time than the first.

I put together a longer checklist version of this over at rHomeLab.com if anyone’s curious (linking since it’s directly on-topic, not trying to spam), but the short version above covers the big ones.