Unfortunately (maybe not), there are no setup toggles to fully automate purchasing. However, you could use standard Job Queues and some codeunits to achieve this.
First, create a Job Queue that uses a codeunit to run your Planning Worksheet overnight.
Second, create a Job Queue to use a codeunit to handle the results in the Planning Worksheet. If your replenishment is pure stock policies (Fixed/Max Reorder) it would be simple. If in the order based policies, what to do with Warnings/Emergency messages? What about Cancel/New combos? What about Date Changes or Quantity Changes? The end goal would be for the system to make the same decisions a person would logically make and then create the Purchase Orders.
You could (should) stop there to avoid sending auto-POs to your Vendors.
However, you could also build in that the POs are auto-Released on creation from the Planning Worksheet, and a Job Queue to send Released POs to your Vendors. At this point you've completely automated (for better or worse) your purchasing process.
I do not agree with everything listed above, but it is possible.
Best Regards,
Ben