Way to involved a topic, need pictures and face to face time and trial and error. A high level snapshot, the best palce to start to get an understandign of what you should look for...
Since we drive road cars or at least track cars based on road cars we are never going to get the sort of ride heights you need to get real downforce from under car aero. So all we can hope for is to eliminate postive lift.
Numbers vary, but its not unusual for at speeds of around 180km/h you can have 40-60kgs of lift. You want to get that to zero or even to the point where you are getting -20kgs lift. So you need to look at airflow.
The heat is an issue, but consider how do mid engined cars cool engines? They are predominantly cooling the engine using radiators and coolers for engine oil and coolant, they are not cooling the engine or exhaust. Though you need to keep an eye on that as well for obvious reasons. Good bonnets for radiators and sides of bumpers for oil coolers excel here
As for the rest of it, there are plenty of good texts around and its too hard to explain without pictures graphics. But seriously, if you read up and get yor head around Bernuolli equation then you are half way there.
Apllied to cars and the topic, basically the Bernoullis equation says that if air moves fast it has less pressure. If air moves slow it has more pressure. The classic example being the blowing air accross the top side of a sheet of paper making it rise. The higher velocity air you are blowing accross the top surface creates a low pressure zone. The pressure on the underside of the paper is higher so the paper is forced upward by the higher pressure.
This is obviously the opposite of what we want. So we want air to flow quicker under the car then over the top. The pressure on the upper surface of the car where the flow is moving faster is lower than the pressure on the lower surface, giving you lift like a wing.