Attached are the main elements for a vehicle ranger which I designed, built and tested (it worked). The unit was contained in acrylic which I fashioned and shaped in our kitchen apartment while doing PhD at W&M (note: acrylic doesn’t like 400 deg F for more than 5 minutes 🙂 ). Control is via a PICmicro 16x, also included is the assembly code for programming the PIC using MPLAB. There should be links somewhere on this site to homebrew PIC programmers, you also need a UV light source to clear the PIC 16Fx if trying out different codes. Best to source the stepper motor and larger things from allelectronics in CA, I use digikey for smaller items.
If nothing else this project will ram home the difficulty in calculating the distance to a moving object using less than 100USD worth of parts 🙂 Nice description of stepper motor types here