Hi,
I have done a lot of fixed-layout ebooks and print books and all three of your formats would be perfectly valid. I would suggest 12pt as the basic type size with line-spacing of 1.4 if they are text-heavy books. It could be very different if they were picture-based books.
You could produce just one version, which would be the print paperback, and then also upload that as a fixed-layout ebook. Basically, you would need to set the paperback size to something that would be compatible with both print and digital devices, so something like A5 should do it. You could then output the book to PDF, fixed epub, and imposing it as two PDF pages on an A4 page.
If you are planning on getting paperbacks printed then you would need to make sure your page sizes and margins were compatible with whatever printer you’re using.
Probably the best plan is to set up a few pages as a test and try out a workflow, particularly with regard to typeface text size as you might decide that, say, 14pt would be a better fit for your projects.
Obviously you would need to follow a different direction if you were going to do a reflowable ebook.
Hope this is of some help.