I don't do any of the resin casting, that's done by talented people who are good at it, and have been doing it for years. I create some custom parts like wheels, tires, alternative front ends (to change my '74 Dodge Monaco to a '77 Dodge Monaco Royal) and get them 3D printed. I use ProEngineer software because I use it at work all the time, then upload the 3D files to somewhere like Shapeways or iMaterialise and they send me a master model.
This gets used as the master model for the silicone mold
I've had several master models printed for resin casting, as said above just make sure the resolution (number of triangles the stl file is saved as) is high enough and you specify a high-detail material for printing.
I've had wheel inserts done, tires and complete bodyshells
Here's the idea
And here's the reality, all parts done from 3D masters, including the wheels and tires for the pickup
For the 3D files I either bought it or created it myself, then uploaded it and did the usual route for silicon mold and resin casts. The tires are urethane, what doesn't come across in the photos is the tread pattern and sidewall letters I could do!