A couple years ago I built a compute server from old parts like this. I looked at old Opterons, but ultimately went with Xeons (4x E7-4880v2) because of HyperThreading, even though they were a bit more expensive (I think I paid $90 per CPU). I was super excited that I could build a 60 core/120 thread computer with 12 TB of disk space and 512 GB of ECC RAM for under $2k.

That excitement lasted until I actually got the parts and built it. Heavy iron like this should not live in your spare bedroom. The 4U server chassis weighed about 150 lbs fully built, and it made over 95 dB of noise. The noise was so bad I couldn't leave it on overnight, and only powered it up to run code for a few hours at a time.

But in the long run, it scored 81k on GeekBench 4, which was over 3x more than my gaming PC, and having 512 GB RAM meant my simulations never needed swap, so they ran several days faster even though the memory was only DDR3. It was a fun experiment, but don't buy an old Opteron CPU unless you have experience with servers and know what you're getting into. It doesn't make sense for businesses to build a system with old parts, and a lot of homelab hobbyists are going to be disappointed if they think they can build something suitable for home use.

Also: If you're wondering why I didn't use AWS or other cloud compute, it's because the costs of running a 32 core instance (which was about the same speed as my 60 older cores) would have been over $10k with my usage pattern- $30/hour for a few hours each day for a couple years really puts a dent in your wallet.