For reference a speed trap is the single camera marker on the map, make sure you are aiming for that and not a speed zone, which is the double camera marker. I show what it looks like and the location in the video.
I personally used the Gold Mine Speed Trap at the far west end of the map, the Redstone Gold Mine, as it is very straight and easy to reach the 135 MPH/ just over 217 KPH for 3 stars. I used my 2014 Porsche 918 Spyder upgraded to S2 998.
The Porsches available are DLC only but if you have done one of the previous challenges that gave you a Porsche it will be in your garage. I counted 22 in my personal garage from previous challenges and I never purchased the DLC so hopefully you have one of those available to you to accomplish this. There is a DLC pack you can buy of the Porsches but it still let me purchase one though the auction house when it usually flags you if you don't have the DLC. I am hoping someone without a Porsche can post some extra info so I can pass that info along.