What are the benefits for me?
If there is none economically (ex I don't recieve transaction fees) what are the actual minimum system requirements to do that?
I'm planning to have a 24/7 node working but I'm scared of electricity costs (in my country it's very expensive) so I'm considering on how to do that with the lowest expense possible.
I own several computers so I wouldn't need to buy hardware, but they either are very outdated or not really powerful (cpu and gpu wise)
Longsword94, Bitcoin's security model
is that you run a full node.
The economic benefit of running a full node is that you know that you are synced with Bitcoin.
Currently, all models for using Bitcoin without running your full node, mean that you trust - you guess - that the Bitcoin ledger someone gives you, is correct.
In a Bitcoin wallet model that does not involve a fully verifying Bitcoin node, some reduced security mode is used. For instance header-only sync and SPV proofs. An SPV proof can prove to you that a payment has happened. But, an SPV proof cannot prove that Bitcoins you believe you own, have not been consumed. It would be expensive to trick your computer which does header sync, however it can be done. So again, running a fully verifying Bitcoin node is the only way to know that you are on the real, valid mainnet chain.
So that is, it's a cost that you pay to know that you are in reality, similarly to how you pay for a newspaper to know what's going on in the world.
Regarding operational expenses and suitable hardware for a fully verifying Bitcoin node:
As pointed out by others a Bitcoin node is a very cheap piece of hardware, except for that using an SSD is greatly helpful. For the computer part, some cheap 64bit Raspberry Pi computer is enough really.
The computationally intense tasks a fully verifying Bitcoin node performs, are 1. to check a lot of SECP256K1 cryptographic signatures and 2. to compute a lot of SHA256 hashes. At the end of the day these operations are still not many, so again a pretty primitive computer will do.
A very high end could maybe sync the whole Bitcoin blockchain in 15 minutes using LibBitcoin. In your case if the initial sync takes days, it's still fine as your timeframe is to run it for years.
This kind of low-power, simple computer fully verifying Bitcoin node should operate in the 0-30 watts total power consumption interval. That should mean 10-20 kilowatt hours per month power consumption. I wonder if anyone has benchmarked this? Or noone did because it was so cheap noone really cared. The energy consumed eventually becomes heat.
Rarely bandwidth consumption could be an issue and you can configure that away.