Mini PC if you want small hardware that can be bought for cheap second hand, my recommendation goes for HP;
ARM SBC if you’re crazy about power consumption and you don’t care about having potentially unstable systems and/or spend the same as a second hand mini pc in extras like a case, power supply, adapter for this and for that;
DIY build with a micro-ATX or micro-ATX board if you’ve the space for it. You can a very powerful machine for less than. Check one of mine here lemmy.world/comment/2676457 ( i5-7400 + 8GB RAM + Board for 70€ second hand)
To be totally direct, SBCs are cool but are a waste of money. You won’t able to get anything with a decent CPU for less than 120€ considering all the accessories they require and at that price range you can get HP or Dell mini PC’s (i5 6th generation) that are WAY more stable and powerful with everything out of the box. Those machines can be found with mobile CPUs so they won’t waste power.