For a sequence of choices, is it better to ask all on one page, or break them up into seperate steps?

Protoype out the different directions and do some user testing with users who match your typical customer. Make sure to have them go through the entire flow so you can accurately represent their mental load by the time they hit checkout. You might even want to try putting the configuration part in a different part of the workflow.

In my opinion, either option will work, it just depends how they are presented to the user.

I prefer the single page design, but that's without more context of the project.

For example, having a combobox for a language next to a combobox for the physical unit would be no problem if there are few options.

If there are hundreds of options, it may be best to separate these two more, which depending on screen size for the application to be used on would vary. Small screens like phones, multiple pages may be a better idea.

Another option is using a modal over top of the current page to make selections in, it would simulate another page without the need to load another page.

The breadcrumbs don't have to change

I understand that one page is the easiest to implement. My advice would be to create a prototype this way and do some hallway tests or test it on potential users. If the form is really long you will almost likely come to the conclusion that breaking it up is preferred. Long forms often perform better when broken up; whether it is because it reduces cognitive load and takes users less long to fill-in or make fewer mistakes, that can only be concluded when it is tested.

Second, breaking a form up to reduce cognitive load doesn't mean you have to introduce extra steps in the process. You can group form parts together under tabs or categorized sections that can be shown with a click etc.

At last I want to give the advice to ask the right questions and have realistic requirements, because you are probably stuck on your definition of what the "advantages" are. Fewer steps on the screen isn't immediately an advantage for your users, but it can be if more steps is becoming a problem. "The breadcrumbs don't have to change" sounds like a requirement made-up by a developer. Fewer things to understand on a screen can be a good requirement when it is made specific and can be proven that more things are problematic.