Does the chosen chassis affect the available job offers?
Solution 1:
Okay, I downloaded a player profile with some play money (1 billion Euro, all skills maxed) and did some testing. I bought the smallest cabin and chassis and took a screenshot of the available jobs. Then I upgraded to the medium sized cabin, took a screenshot again, upgraded to the large cabin, took another screenshot. Same with the chassis.
Afterwards I compared the screenshots. The result: the jobs were always the same. So I guess the switchable parts are really just for show.