I just added automatic bed levelling to my old Printrbot Plus v1 (1303) from 2012 with a piece of glass (mirror) as printbed.
I bought the Mini Differential IR height sensing board for £27 inc shipping, designed a small bracket for attaching it to the extruder / hotend assembly, and connected it to the unused E-stop connector on the Printrboard.
The sensor itself does not work on mirror surfaces, so I attached pieces of electrical tape at two corners and one side of the printbed, and programmed the Marlin 1.0.0-RC8 firmware to do 3-point bed level measurements on the pieces of tape.
Now I just need to do G29 in my g-code after the G28 for homing before each print, and it runs the bed levelling procedure. I have only had it running for a few days, but as far as I can tell it works great!