You will find in this topic short explanations for some of the settings available using credits as well as a few ideas to optimise the workflow.

Camera Position
To change the position of the camera, tick the box Lock Camera to View. The camera will follow the viewport.

Depth of Field
Use the picker tool located next to the distance field to select a part in the viewport. All the parts in the scene located at the same distance will remain sharp. The sharpness of the other pieces will depend on the distance to the focus point and the aperture radius parameter. Select a small radius to slowly blur the parts when they get further from the focus point. Increase the radius to increase the blur power. Generally radius values between 0.5mm and 5mm are acceptable depending on your distance and what you try to achieve. Remember to re-use the picker tool every time you change the camera position as the distance changes.
See more information about the depth of field on wikipedia.

Part defects
Three options are provided to increase the photorealism: Wear & Tear, Discolouration and Flatness Distorsion. Keep in mind that updating any of these options will increase the render time. My preferred values are: Wear: 25%, Colour Shift: 10% and Uneven Flatness activated. You can decrease the wear parameter to 10% if you want to get a cleaner look.

** Randomise**
This value randomises the location and the rotation of the parts to mimic an assembly made by a real person - Parts would not be fully pushed and not perfectly aligned. With high values, some parts may intersect.

Compositing
Contrast, Brightness, Hue and Saturation are pretty self explanatory and similar to any image processing software. As a personal preference, I like pushing the saturation 1.05 or 1.1

Dispersion definition in the Blender documentation is as follows:
This simulates chromatic aberration, where different wavelengths of light refract slightly differently, creating a rainbow colored fringe.
Very low values shall be selected. I generally set this parameter to 0.02 or even lower.

Fog Glow is sort of an atmospheric haze around the image. Set anywhere between 0 and 100% depending on the mood you wish to recreate.

Vignette parameter creates a black halo in the corners of the image. More information on wikipedia.

Floor
To know:

• The circular floor is rendered with blurred edges so don't hesitate to make it big.
• For square floor, you can choose to make it a shadow catcher. Only shadows casted by the plan will be rendered. The plan itself won't appear. This is a good option to use with a coloured background. Increase the roughness to 50 to 100% to avoid really sharp edges for the shadows.
  These options are also available for free renderings.

Preview
What I recommend for a preview image before launching the final render is to use the percentage field in the render tab. This field directly impacts the width and height of your image that can still contain your desired final resolution. If you use 50%, the render height and width will be divided by 2 and the render time divided by 4.

Don't hesitate to reduce the sample size. Divide the value by 2 and the render time will be divided by 2. For testing, I would use a value between 50 and 100 samples.

Deactivate all part defects. This is generally not critical to preview the scene and the render time will be almost divided by 2.

Remember that the cost of the render is directly related to the render time and if you follow the advice above, the preview image will cost you a couple of cents.