Make it printable:
I’ve got a model in these formats:
.3ds, .fbx, .max, .obj and .stl
The model itself was probably not meant for 3d printing. Is there a way of making one of them printable?
@Nathan_Walkner And you think the modelrepair will do the job? Then I’ll try. 
If you have this model in STL and it’s manifold/watertight, you might consider to print it with water-soluble supports to ensure good form overall. Another option, albeit expensive is to print it with an SLS powder printer, which has no support complications. It is good practice to have a design that either “drains” itself or has drain holes designed after the fact.
The greenhouse of the automobile model, for example would be considered a location in need of a drain hole for the powder to escape. If printed with open window area, that problem is solved.
If the glass area remains enclosed and is printed on an FFF/FDM printer, it’s going to be all one color, obviously.
For the detail represented by the wheel spokes, you may have to scale up the model substantially or aim for SLS or SLA printing. SLA printing also needs drains for the uncured resin, as well as non-soluble supports for the challenging areas.
This might require quite a bit of work to ensure that the wall thickness of the body is adequate for the 3D printer that you plan to use.
How much flexibility do you have with modifying the model? If the original design has detailed axle mounting or wheels simply floating in the wheel wells, the treatment would be different for either case. With the correct amount of massaging, you could have wheels that rotate, such as that on a child’s toy.
If you are game to learning a new application, you have the option to use Blender and/or Meshmixer, among others, to make appropriate changes to permit 3d printing.
If you have the files, chop the model up into subsections that can be printed individually, maybe even in different colors, then glued/build it like a real model car kit! The area under the hood, trunk, seats can all be hollow or you could rig up some steering… Maybe work it into the OpenRc project? Lots of possibilities. Give it life!
Or, just once hunk of ugly lifeless plastic. 
Models designed for rendering are generally unsuitable for 3D printing because it’s just shells with no enclosed volume. I heard that Meshmixer can thicken shells like that though haven’t tried yet.
Super
@Fred_U I just need the hull. So I would like to combine all the outer parts. But that turns out verry tricky…
I hope that when you succeed with this project that you will share the results in this forum. A photo of the final project would be enjoyable to see.
@Fred_U Will do it if I succeed.
I did not expect an easy process. But what I’ve experienced is a headache job. 
I’m giving up.
Some parts of the model were solid, some just a mesh and it turned out like a mess.
Thanks anyway for your advice. 
Is the model proprietary or something that can be shared with others?
It’s a model from cgtrader ($). Otherwise I would have shared it up front.
As expected.
In 3D printing there are very few shortcuts.
Had you watched the tutorials at the very least you would know ‘why’ it didn’t work, and with the ‘why’ comes the ‘how’.
@enhydra
I figured it might be a paid model. That sort of detail means a talented model maker.
I checked out the web site and saw that representative models were in the hundred dollar plus range, yikes. There’s a free download of a VW bus, but it originated in SketchUp, known to be unfriendly for 3d printing unless otherwise manipulated.
Before you surrender, consider the following. Load the model into Meshmixer, use Analysis, then Inspector and allow the program to auto-repair the faults. All you’ll lose is time and if the resultant model meets your requirements, you’ve come out ahead. The automatic method makes for interesting results sometimes, closing gaps with peculiar planes.