I'm not sure that any of these bikes are dependable enough for a business environment. Well I guess they might be OK if you're delivering pizzas, but not as a courier. After you broke down a couple of times I'm sure you would drop down a bit in the list of who to call for delivery, especially for important documents pegged to a timeline.
But maybe that isn't in your requirements. If I were in the courier business, I would emphasize dependability of delivery.
In the price range you stated, I would think a Whizzer or equivalent would suit you. I'm sure you've taken tax advantages into consideration. This would be a business expenditure? Maybe our tax dollars would also paint up your bike in company logo/style?
Then build your zippy bike for pleasure.
Did you consider space requirements of an engine and SBT Shifter in a full suspension frame? It can be done but will require modification. Maybe I missed some bikes in this category but I don't know of any where the package will just slip in the frame.
The Shifter does lend itself to full suspension but it is the derailler at the rear wheel that allows travel along with chain management. An additional jackshaft incorporated into the pivot of the rear triangle works very well, too, if you're set on the Nexus.
Welcome to the hobby and forum and good luck with your project. Of course (especially with a full suspension shifted drive bike),
as you progress.