Must fight temptation to buy an overpriced raspberry pi

You lose the I/O and power efficiency is no comparison. You can get better power efficiency and sometimes some I/O with an old router and OpenWRT, but you'll be in the class of a Beagle Bone and a much harder learning curve. I've never managed to get a sensor or peripheral working on some old laptop's SPI or I2C buses like how easy it is on a Rπ.