I think that's a great idea. Here is some general information to think about.
Print books are typically uploaded as print-ready PDF files. It will contain your title page, copyright page, contents, index, bibliography, if you have one, etc. Each page is formatted exactly the way you want it to print out, in the same size as you want your paperback to be. There are a number of standard sizes to choose from, each with specific margin measurements and number of pages allowed. It's helpful to have page numbers, and a good index. If your current PDF file meets all of that, you're probably good to go. There will be checks after you upload that will alert you to printing problems that need to be corrected.
Photos need to be prepared for print, which I'm sure you already know is a different animal than for a screen. Also, B&Ws tend to print out darker than they look on a computer screen. I've never done a color book so I don't know about that. Either way, it's always a good idea to order a physical proof before final approval. It's amazing how different it looks on a physical page, and how your eye can find things you missed on your computer!
The paperback cover file is uploaded separately. After you have your page size, number of pages, and paper color chosen, you can request a cover template. It will be the exact size you need to print out properly and will specify margins for the text and bleed margins for photos. For a paperback, you need both front and back covers, which are designed as one large sheet. Since you don't have a lot of pages, you probably won't need to worry about designing the spine too. I definitely agree that color is a good idea. You have a nice front cover design to start, but color will make it stand out to potential customers.
Alternatively, you can use the KDP cover designer, which I've never tried. Someone else may have more information on that.
eBooks are formatted differently because Kindles can be different sizes, or readers may want to change font size. So the pages are more fluid. I've usually uploaded these as .doc files and then let KDP convert them to mobi. The nice thing about this, is that you can view it immediately on their previewer, make corrections to your file, and upload a new one then and there. You typically don't need page numbers for eBooks, nor an index, since Kindle has a search function.
KDP also has free software called Kindle Create that will convert a PDF file to Kindle. Since you already have the PDF, that may be a good option for you.
Something else to consider is your ISBN. You can provide your own ISBN or you can get a free one from KDP. The advantage to buying and providing your own is that you are listed as the publisher. Otherwise KDP will be listed as the publisher, which means you can't later take the book somewhere else to publish, for example your print book to Ingram Spark or your eBook to Smashwords or Draft2Digital.
Whether you do Kindle or paperback first, you will be given the option to turn that version into the other edition. I wouldn't recommend that because of the formatting differences. I have a couple of excellent nonfiction paperbacks that started as Kindles, but as paperbacks they don't have page numbers nor an index. So it's impossible to find useful information in them again, which makes the books less useful. One of them really looks wonky as a paperback, which makes the book look very amateurish. That's a shame because it's really a well-written book.
If you haven't already, check out KDP's Jumpstart. It will run you through the process and give you an idea of what to expect and how to do it.
One thing to be aware of is that Amazon print on demand has set minimum margins for each size of book, so if the margins in your PDF are too small, and text might get cut off in the printing process. They also have guidelines for cover design, to allow for a margin of error in printing so that nothing important gets cut off.
It might be worth downloading their cover and interior templates and comparing them to your existing files.