In order to be able to equip all the top end performance parts you will need to have reached level 50 REP, details here Can I Get Your Autograph? in Need for Speed
and have completed the Build storyline with Amy.
The top end parts are all the orange E
performance parts, there are 21 different parts that you will need to equip. From what I can tell the parts cost about the same no matter how expensive the car so pick whichever you want. You will likely be upgrading as you progress so you wont notice them costing a lot.
You might earn some free parts from the pickups around the map though the highest one I got was a top level Forced Induction which is unlocked at REP level 40 anyway.
Here'd a video showing each part upgraded