In general cars must keep the aero fixtures from the homologation car. No exchanging a spoiler for a wing for the race, no addition of lips. If you want a wing, it must already be on the homologation car.
I will clarify later that you can increase downforce up to Group 2/4 limits when coming from Group 1/3 using the aero sliders. Does that answer your question?
Regarding engine reliability: No recommendation. The more, the better. It can always hit you, regardless of reliability. I won’t tell any failure chances, as I still need to balance the model for a 24h race. That won’t happen within the next few days.
No car will retire completely because of any failure - every car can be brought back to the pits and be repaired. But it might take a looong time if you had major issues. So, the recommendation is again: as much reliability as possible.
Edit: I’m away for the weekend and won’t be able to provide homologation confirmations to the very early birds during those days.