I do have an old laptop and I would like to know the minimum specs for running a Substrate node. This node should be also a validator.
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There are no hardware requirements for running a Substrate node itself, but for running a Polkadot validator or collator.
You can toy around with a local Substrate network by appending the
--dev flag to your node startup command.
The Polkadot reference hardware is described in the wiki.
An "old laptop" is most likely insufficient for running a validator. Keep in mind that a validator will loose money for being offline. Especially when you enable nominations and thereby risk getting other peoples funds' slashed.
The 1KV program on Kusama is meant to help onboard new validators: https://thousand-validators.kusama.network/#/