The simplest build is going to be bench-mounting a motor and adding a chuck. See here: Bench motor

For sure belt sander will top the list. The nice 2x72s are thousands of dollars new, and the most expensive component, the motor, can be salvaged from a free treadmill. Salvage the speed controls too, to control the belt sander motor speed. Also salvage the incline motor from the treadmill; it can be used to tension the belt. Along the same lines: weightlifting benches can provide frames for tools.

Here's a good video on wiring salvaged motors: