Look for second hand business PCs. They can be had extremely cheap and draw almost no power.
I have a Futro S920 and an HP ProDesk mini G300 (or something like that, never understood the naming). Both draw about 5-7W from the wall, which is roughly the same as my RPi3b.
The HP one has an i5 6500T and 16GB RAM, both upgradeable. There's also a real 2.5" slot including SATA connector on the board, an M.2 slot and an NGFF slot. Really cool device. The only slightly annoying thing is, that the fan can't be controlled (or at least Debian can't detect any PWM devices), and it's always slightly-on, which can be annoying. But for 100€? I think that's a good deal.