Hello,
To quote the unnofficial 2021 iGP Manager guide Section 2A
"Possibly the biggest cause of frustration for new players is car design. With minimal DP generation and large research power choosing what to prioritize is very dependent on those around you. However, there is a generally superior development path in terms of attributes to prioritize.
Acceleration+Handling are most important, followed by
Braking- which is not as important as it was pre-3D but it is still useful,
Downforce and Fuel economy, then
Tyre economy is generally not worth it except at very low values and
Cooling and Reliability are never worth putting DP into or researching.
Try to find a CD with a weakness of cooling/reliability and a strength of one of the big 5 (Acc, Han, Bra, Dwn, FE)."
This quote was copied from
this thread which I recommend giving a read for more information into car design and research.