Just an idea and a few parts for now..
Links:
http://www.microwave-solutions.com/shop/modules/mdu1720.html
Arduino project
http://forum.arduino.cc/index.php?topic=91793.0;wap2
Parallax module with analog front end
http://www.parallax.com/product/32213
Interesting notes about directionality of this type of module(or lack thereof)
http://electronics.stackexchange.com/questions/55501/can-i-improve-the-directivity-of-an-x-band-radar-module-with-integrated-plane-pa
Hacking the Mattel radar gun – it uses almost the same RF part as the cheap modules.
http://www.edparadis.com/radar/
Module for sale with filters