I wouldnt let a stage 3 chip put you off buying the car. TBH you will end up doing that yourself at some point anyway.

Dark green injectors will have 803 on the serial number somewhere and check the map sensor which is under the header tank on the wing. If its a three bar it will be black and grey.

Dont expect the person to let you open the ecu to see the chip, i wouldnt be to happy with that if it was my car. the guy selling should really know whos chip it is.