The only really worrying part may the consumption.
Measure it
Here, prices easily go up to 50 cent (euro/dollar, it's the same nowadays) per kWh. That means 30W costs 11 euro/dollar per month. If not for the disk space requirements, a VPS could be cheaper (but also more risky for running a node).
Back to the 11 dollar per month: that's 400 dollar in 3 years, and could very well justify buying a much more energy efficient second hand laptop to use as a node.
I wish it would be easy to measure. Now the electricity price is subsidized, until next spring at least. There are so many taxes and then reductions I cannot properly measure.
By not-so-strict calculations 4 months ago I paid around 15 cents/kWh, 2 months ago I paid some 7 cents/kWh. This month... I will find out in some 10 days.
Since I expect the 7 cents may be an error, I'll go to a middle 11 cents for my calculation, which (based on yours) means 2.42 EUR/month and 88 EUR for 4 years.
While you did scare me, since I feel like CISC processors tend to be more power hungry than RISC ones (eg ARM), hence a RPi4 would have been better for the job, this setup doesn't look like a bad business after all...
I will do the math again after the next invoice arrives.
This got me thinking: how cool would it be to run a node on a phone? Replace Android by Linux, add a 512 GB microSD (around 50 bucks), and login remotely.
Or even without replacing Android:
stackexchange mentions installing Bitcoin Core in Termux.
Phones take at most a few watt power, and modern ones have more RAM than low-end laptops.
microSD may not be best suited for such load. From my experience it tends to heat up and start failing (but maybe the adaptors I've been using were not top quality either).
And heavy duty microSD are overly expensive. Plus the 512 is just barely enough. I also guess that a more powerful smartphone may be more power hungry too, but I may be awfully wrong.
Interesting research though.
[Also, fyi: that SH system I consider buying, with that Intel G proc, 4GB RAM and 1TB HDD is just a tad over 100 EUR].
than low-end laptops. You may want to turn off data
You should have no SIM at all in it. Then no risk to have data
And I would not take into account systems that have own screen. There's usually no reason to buy screen for a machine supposed to run in unattended mode. Remote desktop - in a way or another - should do.
But this is also because I was talking about buying hardware.
Fun fact: before this thread I was considering to play with Bitcoin core on a RPi zero W (32bit), just to see how bad it runs. Afterwards I decided I should not waste my time.