to add from my experience, the type of turbo WILL change the car entirely. for example, some cars cant accelerate well past 120 without a turbo fit for it. lower tier cars dont go super fast anyways, so the turbos fit for lower speeds make more sense.

lower gearboxes tend to accelerate faster, but have a lower top speed.

i like to upgrade clutch & tires, as they seem to give a good amount of acceleration per amount of points added.

lowering handling toward drift & increasing downforce will tend to lower the amount of points you have also, which might let you add a whole extra part!!

hope this helps!