There are basically 3 separate things, you have to take care of:
- physics,
- graphics (3d model + textures and stuff),
- sounds.
For physics, you need (some general notes, without going into details):
- full suspension geometry (connection points on the body and hubs for all rods, wishbones etc.). It might take from just a few hours to a few days, depending on how good you are prepared for the job. My first attempt with measuring a race car, took about 5-6 hours and I still didn't finish the job. With some road cars, it was like 3-4 hours total and I was happy with the result,
- total weight of your car and as separate, weights of hubs, tires and rims and dimensions, to calculate inertia,
- aerodynamic parameters of your car. The easiest, is to find them on the internet and the rest is a guess work. The best way - take your car for full testing in a wind tunnel

- pacejka parameters for tires the car has. If you don't have them, then if you are lucky, you maybe find similar tires in other mods... but if those won't be good tires, then the whole work will be ruined. Tires are the most important thing here.
3D model - not much to say here. You have to make the model from scratch or use something existing. The more effort you put into the project, the better the car will look.
Sounds - you have to record engine samples. Without a good microphone and methodology, you won't achieve good results. Other sounds, like tires, wind etc. you can get from ISI mods (good samples and are for free).
Good luck
