You will only make a few blocks a day depending on how much you stake. It's not too intensive from what I've seen. I think it will work fine even on a slow HDD.
One important thing to note is that the proof of stake Ethereum chain is a different chain than the proof of work Ethereum chain. You won't need the legacy PoW chain to stake.
With PoS you only need data from the last checkpoint to stake. This is only a subset of the entire chain. You also only need the data for the shards you are a validator for which further reduces the data required. The goal is to allow anyone to stake if they want to regardless of hardware in order to increase decentralization.
