Can I create an orchestration with just one stage and one step?

Yes, an orchestration can have only one stage and you can add just one step to an orchestration stage. If a step requires a success step, then add a Stop step to satisfy that requirement. The only steps that don't require either a success or failure steps are the Next Stage and Stop steps.

If the step is a manual step (one that requires action by a salesperson) then you must also add one objective to the orchestration stage and map the step to it.