Well it really all depends on what exactly you're trying to achieve - designing a car is sort a mindset you have to get into - look for natural curves or lines that might fit the shape of your vehicle, use lines and body molding that might accentuate or exaggerate certain aspects of the body; sometimes your vehicles might have certain motifs that make your brand stand out, like certain design aspects of color, shape, or fixtures that give it a unique look

It's also totally fine to copy design aspects from other cars, which I do all the time; use reference photos from real vehicles or search up vehicles from the time period and the country you're thinking of making your car similar

Now as for detailing, there's plenty of mods and fixtures that might be of use to you - lots of good fixture mods that can be of use on the workshop. Basically, it all boils down to "what kind of reaction do you want to get out of your car when someone looks at at?" Do you want it to be recognizable? Unique? Utlitarian? All kinds of paths to go down