Emon Crew

I was wondering if anyone knew any sites that gave out car history reports for free? I know CARfax does it, but they charge you and I've heard that often times carfax tends to leave out stuff. I ask because my little brother is in the process of buying a WRX that's listed as a non-STi with 18000 miles on the frame, however the owner has swapped in a new STi motor which he says has 2200 miles on it. Does anyone have any advice on this kinda situation? Should I be checking the VIN #'s for the car and for the motor? How do I know if everything is legit? Any help would be greatly appreciated. Thanks in advance