I'm sure you will get a lot of opinions on this! I'll wade in with mine.
To provide services to a large potential client base you will need tools that can read the documents your customers send you with little or no conversion. Tools that can also handle your flyers and brochures as well as books. Tools that can create the different formats that you will be dealing with.
MS Word(or Open Office) combined with Sigil and Amazon's Kindle Previewer will give you all of these capabilities.
You will get most of your manuscripts in Word. Layout is easy and versatile. From the Word document you can create a pdf version.
Saved as a filtered web page and imported into Sigil will get you the epub version.
Run this through Kindle Preveiwer and you have the Kindle version.