Any downside to reusing a domain controller's name and IP?

Reusing both is workable, but the cleanup and replication checks matter. Transfer any FSMO roles first, demote the old controller, and verify that its domain controller metadata and DNS records have been removed. Check the relevant service records as well as the host record.

Wait for the removal to replicate to the other controllers before bringing up the replacement with the old name and IP. Replace one controller at a time so the others remain available.

I'd reuse the IP if needed, but give the new server a fresh name. It makes the old and new machines easier to distinguish and reduces the chance that an obscure service or a device that has been offline gets confused by the reused identity.

That's a preference, not a claim that reusing the name can't work. You'll still need to find anything configured to contact the old hostname and update it.

Keeping the name and IP can save work when appliances or other systems have a specific controller configured instead of discovering one automatically. The tradeoff is that you need to be thorough about removing the old controller's records before introducing the new one.

Another option is to build the replacement in parallel under a new name, promote it, and transfer any roles before retiring the old controller. That gives you a separate new machine to check before demotion, though you'll need to deal with references to the old name and IP.