Need help with decryption rig

[Sondero]

It depends on what you want to crack.
On some algos it‘s as fast, or faster than a rtx2080ti, for much less money.

Somewhere i red that min. Is 1-2 Cores for your OS an 1 physical Core for each GPU.

Will be decrypting SHA256

Which kind of data exactly (which hashcat mode) do you want to decrypt  ?
But for Sha256 GPU´s will be the best option.
You can google " -grafikcard   hashcat benchmark" to find a benchmark for most of the actual GPU with hashcat.
With the hashes/sec and the price you have to pay for a GPU you can decide which GPU is the best for your rig.

But dont forget, if you use newer GPU (expensiver, and not so much GPUs) you need less power supplies, less head, easier to expand and better to sell if you dont need it any more