why not forget the low end motion sensor stuff and just record a steady feed to a PC with some camera's with decent image quality. Motion sensors just make everything expensive and have more room to fail or be thwarted. without decent camera quality you'll be able to see someone messing with your car but you won't be able to prove who it is.
Motion sensor floodlights you can get at the home depot for $30 if you want that feature to drive off would be vandals/crooks.
http://www.homedepot.com/prel80/HDU...ID=cchkaddjlmhdijdcgelceffdfgidglo.0&MID=9876
Just get a cheap PC with lots of hard drive space and set it to overwrite old feeds after the drive gets too full...the camera you pick will probably come with software to do this.
Isn't there someone on this forum that did this last year at his apartment? I can't remember who