Your own calculation is the thing that should pick the machine. At 0.35 EUR/kWh a single watt running around the clock costs roughly 3 EUR a year, so the gap between a 25 W old laptop and an 8 W mini PC is about 50 EUR a year, or 260 EUR over five years. Half your hardware budget, paid in electricity.
That's also why cheap used enterprise gear tends to be a trap at German prices: low purchase price, 40 W at idle. An N100 or N150 box with 16 GB and an NVMe drive sits around 6 to 10 W idle and fits inside 500 EUR comfortably. On the storage side, two 3.5" disks add something like 8 to 12 W if they never spin down, so spindown is worth configuring when access is bursty, and if your data fits on SSDs the power question mostly goes away.